I agree with Cae here. Making it single-threaded only was a deliberate design decision. The Boost C++ libraries that support smart multi-threading with threadlock barriers and whatnot were not even a thing by the time BGT was first released to the public if I'm not mistaken, and even if they were, Philip didn't want beginners to have to struggle with yet another fundamental programming concept that becomes second nature once mastered but is really difficult and challenging for most novice programmers and/or scripters if you will who haven't had any previous experience. Just look at how many people are still struggling with something as rudimentary as handles, although I do agree that the way they are implemented in BGT, or rather Angelscript itself, is somewhat counterproductive and non-sensical. :-)
However, all this academical stuff aside, I haven't yet come across a single instance during the development of my games where I would need to implement something that I couldn't do because of BGT's single-threadedness, if that's even a word. :-) Yeah, some things could have been done more elegantly or efficiently if multi-threading was an option, but to tell the truth, whocares about code elegance or performance efficiency in the context of solely Windows-based audiogames without taking advantage of any truly modern computer features, as someone called it? If it works, doesn't crash and has no gameplay breaking bugs, I'm fine with it, given the scope it's intended for.
The only real, practical, every day problem with BGT not being updated any more that I can foresee is that games written in it will sooner or later become less and less reliable, eventually leading to the point where they will just no longer work on modern versions of Windows at all. Even if Microsoft is painting the thing as just a single universal OS by the name of Windows 10 to rule them all, the fact is that every major aniversary update is essentially as huge and complex internally as what service packs for prior Windows versions used to be, and some could probably even be called entirely new iterations of the system. If not now, then it will certainly become the case at some point in the future. Not to mention that if it was still being maintained at least on a semi-regular basis, maybe something could be done about the Windows Defender false positives, and improved networking capabilities and DLL support could also come in handy in relatively many cases that *are* related to game development.
Even so, I do not consider the current state of things a major drawback until actual serious problems start appearing. I've noticed that only games that do utilize an internet connection of some sort seem to get flagged by antiviruses. A game that doesn't touch the network object at all seems to remain unnoticed. And if your game does use internet, like Manamon, Super Egg Hunt or Oriol's games, creating an exception for it is not that hard after all, as long as you are using an accessible antivirus. So, for the time being and for my needs, BGT is still a good starting point for me, at least until I finish all my currently ongoing games. When I'm done with them, I can start worrying about the future, before I start developing any new projects.