Allowing only bug fixes into branch

Hey folks,
I maybe have a bit of an esoteric question. But : I have this main branch where we have 150+ engineers contributing to a monolith. This monolith naturally piles up a lot of bugs.
As part of improving our processes and code I wanna temporary forbid people to merge onto branch’s except they are not merging a bug fix ideas how can I limit this? I was thinking if I’m able to allow MRs containing links to bugs only but I’m not sure if this is the proper approach .