Frankly, I fear that it is all but dead, even though organizations and even journals continue to exist that have either that name or its close relative, regional economics, in their titles. However, I am teaching a course called Regional Economics right now, as I have for decades, and there has not been a textbook bearing the title of either "Regional Economics" or "Regional Science" now for several decades. Yes, there is one by Philip McCann just out in a second edition from Oxford University Press, which I am using, titled "Urban and Regional Economics," but that is what it as been reduced to, a tail on the dog of the definitely thriving Urban Economics, not to mention that it is a book published in Britain. But as for the broader "regional science," it has gone the way of the dodo, I fear.
To a substantial degree "regional science" was the invention of a single man, Walter Isard, whose death in 2010 at age 91 may have been the final death knell of the field, even though he had himself drifted from it several decades earlier into another invention of his, "peace science" after he moved to Cornell University in 1977. He founded the Regional Science Association in 1954, drawing on the older location theory coming particularly from the German language tradition that began with the 1826 publication of Der Isolierte Staat (The Isolated State) by Johann Heinrich von Thunen. This was followed by the establishment of the Journal of Regional Science in 1958, which still exists (I published a book review in it a couple of years ago), with Annals of Regional Science starting in 1967, and at the behest of Isard in 1971, Regional Science and Urban Economics, perhaps the most respected of these journals today, although this may reflect its having that "urban" part tagged onto it. Others began as well during this period, but most of them are thoroughly obscure by now, such as Papers in Regional Science and the International Regional Science Review.
Also, in 1956 after Isard moved from MIT to the University of Pennsylvania he founded the Regional Science Department, which began granting PhDs in that subject (I note that he served on the dissertation committee of my major professor, Eugene Smolensky, who got a PhD in econ at Penn). A sign of the decline of the field came with the closing of that department in 1993, mourned publicly by Paul Krugman, whom many had thought had given the field a new life with his work on the "new economic geography" a few years earlier, which had been preceded by similar work in RSUE by Masahisa Fujita. And while their approach received much attention during the following decade, including with the excellent book they coauthored with Anthony Venables on The Space Economy at the end of the last millennium, that effort seems to have spread into international trade, economic geography, and urban economics, without reviving or sustaining either regional economics or more particularly the inherently multi-disciplinary regional science all that much (Please note that I am not blaming Krugman for what has happened regarding this in the least).
I am not really sure what happened here, although the disintegration of regional science back into its core disciplines in rump form as urban and regional economics, economic geography, as well as related versions in political science and sociology seems to have played a role. Certainly there are many interesting issues in regional economics in my mind at least, with foundational issues in nonlinear and complexity economics first being addressed certainly in the broader urban and regional economics, with Krugman among others noting this. Many would argue that the foundational paper in agent-based modeling was in urban economics, the 1971 paper in the Journal of Mathematical Sociology by Thomas Schelling on urban residential segregation, a paper and model that continues to attract much study, although again, it is viewed as being urban economics more than regional economics or regional science. The link between regional economics and international economics has long been recognized, with Bertil Ohlin's classic work bearing the title International and Interregional Economics, with some arguing that why Krugman was the first to win a Nobel Prize in international trade since Ohlin was due to his also applying the same ideas to regional economics as did the Swedish Ohlin, even if one of the inventors of what he applied, Avinash Dixit, still has not made that trip to Stockholm as has neither Fujita, who preceded Krugman in applying the same ideas.
Maybe it is because regional economics is a sort of meso-field. With the exception of when people look at multi-national regions such as the European Union or Eurozone, it generally deals with sub-national entities that may or may not be well-defined. But it also tends to deal with entities that are above the very well-defined entities known as cities, where microeconomic issues tend to predominate and with policy issues strongly linked to political entities that are distinct in the form and focus from others due to their greater density of population and reliance on highly developed infrastructure, such as water and sewer systems that may be of less importance in "regions." However, as sub-national entities they do not have the institutions that national economies have, such as those that deal explicitly with macroeconomic policies, fiscal or monetary. To focus on state governments, they do not have monetary policies at all, and at least in the US, their fiscal policies are highly limited due to their facing strict balanced budget rules. They are macroeconomically subject to what goes on around them at higher levels, but lack the peculiar fiscal issues associated with managing urban infrastructure as well. Their meso level nature leaves them in a sort of no-man's land, even though such things as export base multipliers and input-output models are highly relevant to them, with Isard in particular working on the latter as a student at Harvard under Leontieff. It may also be the decline in the use of the latter with the fall of central planning after 1991 that has contributed to the demise of the broader field of regional science.
In any case, when I look around now, it is very hard to see even regional economics per se as a field in the US, much less the broader regional science, although I do not have a definite reason why this is the case, and I personally continue to think that the field, or at least the idea of it, is still important, maybe even of renewed importance, which makes its absence all the more disheartening.
I close by noting that the late Walter Isard may have invented another field, one now labeled as "transdisciplinary," but has received nearly zero credit for doing so. That would be ecological economics, which has a thriving journal of that title founded by Robert Costanza. Costanza and some of his associates view themselves as having coined this term and invented this field, under the influence of Herman Daly, in the late 1980s, and I cannot disagree that they did so independently at that time. However, in 1972 Isard published a book that I not only bought a copy of at the time but still possess that uses input-output analysis as its central tool, which also inspired Daly in his first paper from 1973 in the JPE on "Economics as a Life Science," this book bearing the title Ecologic-Economic Analysis of Regional Development from Free Press, which promptly dropped into complete obscurity. It was Isard's last book on regional science. I note that it initiated what Daly advocated: thinking of the broader system as an input-output one in which there are quadrants of economic-to-economic, our usual I-O matrix, but then ones of economic-to-ecologic, which includes pollution (among other things), of ecologic-to-economic, which includes agriculture (among other things), as well as a strictly ecologic-to-ecologic, with many pure ecologists dating from the work of Eugene Odum in the 1950s using such matrices to study ecological communities. I find it unfortunate that along with the death of his field of regional science, Isard's role in foreshadowing modern ecological economics has also largely been forgotten.