Have you tried asking on their official Discord? That seems like the way I'd try to get it. If that doesn't work I may still have a copy of the installer saved, I'll have to look.
Viewing post in Text Input Template for GB Studio v2.0 comments
Okay, here's the last daily build I have for Beta 4, from 2020/10/11, as well as the most recent version of Beta 5 I have downloaded from 2021/05/22: https://www.dropbox.com/scl/fo/5yc9pgwcwxba9ece1oxq2/h?rlkey=ohbu2pern0iny7tvxmy...
You can see the changes via the Commit list here: https://github.com/chrismaltby/gb-studio/commits/v2beta8.2.5 I believe you can also compile a specific version you may need by clicking on the < >'s for that particular commit, downloading that version, and following the build instructions. I have a bunch of other daily builds from around that era if you need a specific version.