Bookmarks and tab URLs don’t contain porn, generally? References are not typically considered explicit, though certainly their language isn’t clear enough about that.

If you bookmark a collection of data: / blob: links then that would be the outlier scenario where you shouldn’t use any third-party server-involved bookmark syncing service, as presumably they’ll all either break or ban you once they find you using their bookmark table space for data storage.

Good point about Relay.