At least Gitlab (similar to Github pages, I never used Github Pages, always Gitlab Pages) gives you a verification TXT record in your Gitlab Account, which needs to stay in DNS as TXT. So if I used to host hi.example.com on Gitlab (& my own TXT record was hosted, and publicly visible), now I don't own example com, or gitlab account got deleted (but still left DNS CNAME records intact) and scammer gets the domain, when he grabs domain and adds hi.example.com to his Gitlab Account to scam people, his Gitlab Account will have his own TXT record. (now) His hi.example.com can never point to "my" gitlab project or page.

https://docs.gitlab.com/ee/user/project/pages/custom_domains...