If your definition of 'mature' means that the libraries interface cannot evolve anymore, then I guess AngelScript isn't mature yet (even after 10+ years of development). But if your definition of 'mature' is how many successful projects use AngelScript and how long it's been available then AngelScript is really quite mature. Guess, which definition of 'mature' I use. ;)
Most developers that use AngelScript keep a copy of the version they integrate, they even check-in the source code in their own repository. This removes any dependency on my repository and avoids confusion when other users download your project (as it will include the exact version of AngelScript you're using). The developer is then also in full control when he wants to upgrade to the latest version of AngelScript, or if he'd rather continue using an older version. Some developers upgrade with every new release (usually it is just a couple of tweaks to the existing code), others only upgrade every couple of years (and usually have a little more work).
On the topic of version numbering. The 1st digit is the major version. I'll only change this to 3 when I'm ready to take the step to remove old functionality (and breaking script compatibility). The 2nd digit is the interface version. I change this everytime there is a change to the interface (though usually it is just an additional parameter, or method, or similar). The 3rd digit is for releases that doesn't change the interface, but implement something new. On some occasions I've also used a final letter, without changing the numbers. This is when releasing only bug fixes, without any other changes.
I try to avoid changing the interface all the time. Normally I hold off on features that change the interface until I have a couple of different ones that I can release together, but I cannot stop it all together as then I'll soon run out of options to evolve the library. I also definitely will not maintain 2 parallel versions. That would only lead to even more confusion and would take away from my already too few available hours to work on the library.