What's the software running this forum called? Could not find a header or footer talking about that, anywhere....
Software behind this Forum
6 days later
....well, since nobody was able to provide any information, I researched again & found the solution.
https://community.keyoxide.org/d/12-support-indieauthrelmeauthoidc-login-to-the-forum/3
This is Flarum.
Ironically, I have been dealing with Flarum a couple of years ago, so this discovery was awkward. 
I remembered it completely differently. It perhaps improved, over the years.
Case closed.
8 days later
Sorry for the late reply, took some time off. Glad you solved it 
indeed it's Flarum