A bit complex, but there’s a way to stop seeing these messages is by using a special script or creating a small program for your browser. This doesn’t actually remove the message from GitLab but makes it so you don’t see it when you’re using your browser. You can use tools like Tampermonkey to run these scripts. This method changes how things look on your end without affecting the actual GitLab site.
I disabled merge requests under general settings for the repo where I push to develop. This works for me because it’s a private repo with no upstream. YMMV.