Their development branch is very active (last commit 3 weeks ago):

https://github.com/ahrm/sioyek/tree/development

Are you perhaps referring to the latest release (which was 1.4 years ago) rather than the latest commit on main (which was 41 days ago)?

Sioyek developer here. As others have mentioned the development branch is fairly active. I also have my own private branch which I have not published yet until it is more stable, the last commit in my private branch is yesterday (6 commits actually).

Awesome