That's why for that type of 'semi-white-label' thing, since the main risk is one of impersonating the platform owner (like the billing.my-thing.example) or possibly lending the credibility of "our" brand to some rando UGC, I always push for the most boring and generic second-level domain, like if it's the travel business, 'travel-systems dot us' or in edtech, mylearningplatform dot net... Then push all customers who know what they're doing into 'BYO DNS name' anyway.

I also like how sites like github use githubusercontent.com or something like that when linking to UGC assets directly, to avoid someone direct linking to something with the implication that it's coming from GitHub.

It's mostly about not sharing a security context with github.com.