I assume last time game was really stable was when 2.3b was released.
It is also last public release, still found on old develper website.
Originally, any time new release is out, month later old release would go public.
This is even still written on Simbro Patron, but never happened after this point.
Version 2.4 was also done by save dev (it had update notes on his website), but this is point in game when he started work on new zoning system, had some issues and then rolled back most of the changes. Even then, it had some locations moved to next part of the map, and it was really easy to bug out if you press wrong buttons at the wrong time.
After that, all releases where delayed a lot (2.5/2.6/2.7), and some where really buggy without hotfixes (a, b, etc...), and there were no updates anymore on old developer website.