With the recent purchase of Github by Microsoft, the Solarus Team has decided to leave Github. We choose its alternative Gitlab, which we already know because we used it to secretly develop our last mini-game Mercuris Chess, in April last year.
All the projects are still on Github for the moment, but in a read-only state. If you want to contribute, the code is now here on Gitlab : https://gitlab.com/solarus-games.
Be sure nothing will change, since Gitlab is almost identical. Development will work exactly as before.
See you soon!