Oh, I see. I revoked the OAuth app and didn’t lose access to travis-ci.com so I thought it was no longer needed.
But some things didn’t work for me (e.g. "Travis CI - Branch Expected — Waiting for status to be reported" despite travis-ci.com enabled for my fork and "No status checks found" trying to add a branch protection rule despite Travis integration set up) – this may be the reason. So I guess it’s still necessary.