It's quite a common sight if you often download installers from smaller publishers. On MacOs you get similar warnings (even though Mac "notarization"[1] works differently compared to Windows certification[2]) Obviously it's up to you if you want to consider the application trusted or not. My rule of thumb for open source projects is that I trust them if they have lots of "stars", and Bespoke has 1.2k https://github.com/awwbees/BespokeSynth/

(but I'm not suggesting you follow the same rule, I'm not responsible for anything that happens etc. etc.)

[1] https://developer.apple.com/developer-id/

[2] https://docs.microsoft.com/en-us/windows/win32/win_cert/wind...

(edit: added links to Mac notarizing / Windows certification guides)