A couple of examples spring to mind... and the URLs should be adequate in hinting at what the constraints are :)

- https://itch.io/jam/cga-jam

- https://itch.io/jam/fantasy-console-game-jam-3

- https://itch.io/jam/2-buttons-jam-2018

- https://itch.io/jam/procjam

- https://itch.io/jam/1-bit-clicker-jam

- https://itch.io/jam/gbjam-7

- https://itch.io/jam/musicgamejam

If you can think of a constraint, there's a good chance a jam already exists for it. If not, hosting one is easy, and very fun / fulfilling. See https://itch.io/docs/creators/game-jams.