If your IP (and possible your browser) looks "suspicious" or has been used by other users before, you need to add additional information for registration on gitlab.com, which includes your mobile phone number and possibly credit card information. Since it is not possible to contribute or even report issues on open source projects without doing so, I do not think any open source project should use this service until they change that.
Screenshot: https://i.ibb.co/XsfcfHf/gitlab.png
Like others, I had an account before this was implemented. I have a couple projects on there, also mirrored to self hosted gitea. Have had people refuse/unable to contribute to the gitlab project due to the kyc requirement, so I'm thinking I will migrate to codeberg soon.