I think you don't get my point. I'm not advocating for denylisting gmail.com because it produces spam (although this has tempted me on more than one occasion), i'm saying fighting spam in federated environments has decades of experience of various techniques that work well. Open nodes (eg. remailers) have terrible reputation and are denylisted pretty much everywhere, but specific communities/servers can maintain a decent reputation as long as they have some form of moderation/cooptation. By opting into the federation, Gitlab could support various advanced workflows depending on your threat model:

- a new organization using your project? maybe grant their whole gitlab instance "issues" read/write access to the project

- publishing FLOSS in a "community" setting where random people submitting contribution is not expected? maybe we can check the PGP WoT before deciding whether to accept that PR

- running a federation of organizations, some of whom may run their own instance? allowlist all the instances so they can interact across instances

- running a public forge like gitlab.com, codeberg.org, or chapril.org? maybe maintain an allowlist of servers who ask for it and pledge to fight spam

- feel adventurous? setup an entirely public instance and help catch spam and reporting it to denylists

All this is already possible on email level, but pointless as you pointed out as trustworthiness of the mail server is not correlated to trustworthiness of the forge.