This article is a stub. The in-game documentation lists all the changes, however.
ClassicZoo is a fork of ZZT by Adrian “asie” Siekierka, started in late 2020 and initially released on the 6th of March, 2021.
If you'd like to learn more about ZZT, the Museum of ZZT is a good place to start.
The libre version of ClassicZoo differs in toolchain only - it's compiled using Free Pascal, rather than Turbo Pascal. However, as FPC's current (as of 3.2.0) 8086 compiler seems to generate slower and bigger code in general, it is primarily provided for users who seek to achieve free software purism. If you're a first-time or casual user, you most likely want the non-libre version at this time.