Version before last supported 1.16.x to 1.20.x and this last version just says 1.16+ does that mean it supports 1.21?

It seems you are referring the dev version. No, it not support 1.21.

BUT the experimental does. You can download it here:
https://blob.build/project/Slimefun4/Experimental

Unlike dev version, it doesn't support older versions. Plugins that doesn't cover the new API won't work!

Supported 1.21-1.21.2. This will break all addons that do not support 1.21. Use at your own risk, this is still experimental

Update: this is piece of message I got from bot on Slimefun server:

Note: The experimental build contains breaking changes and will break any addons that have not been updated. It fully supports 1.16-1.21.1, and partially supports 1.21.3+ (there are known unresolved issues). Use at your own risk.

It is kinda confusing, to be honest. That message implied the experimental build is working on 1.16-1.21.1 with several issues on dev build solved(?).

please ask questions on the discord. this is for error reports. closing as invalid.