Does anyone else use these kind of trailers like you do?

# See: https://example.com/

# See: Issue #123 <https://example.com/issue/123>

I have been putting the issue number at the start of the title so I could see which commits were related, but it seems like people would not want to do that with a 50-char subject line.

Yes optionally, because trailers with links can be especially helpful when a commit is relating to specific URLs, such a relevant blog post, or design image, or planning document, or industry publication, or committee RFC.