To get a feel of what's in then (in short: tons of stuff, including hardware acceleration with Metal) check here:
https://iterm2.com/downloads.html
under Test Releases, and click "Show Changelog" to see the changelog for the latest 3.5beta7, and "Show Older Versions"to see the changelogs for all 3.5 betas and several 3.4.5 and 3.4.4 betas (all of those released after 2020).
I've used the latest beta as it lands every time (you can enable the "dev release train" somewhere), and didn't have any problems!
Would be nice if GitHub would provide an option to scale those charts so single commits with massive additions and deletions would not overwhelm normal activity.