Ha, I assume you are joking but of course this is a valid point.

If you use your own domain, that should equal self-hosting. Using another domain than your company's does not mean it can't be self-hosted, but it is a strong signal. And AFAIK, Slack does not offer anything like redirecting to on-prem instances from slack.com.

And conversely, there is no way to self-host Slack, right? Which was gp's point.

perfect use case for the public suffix list (https://github.com/publicsuffix/list)

e.g. add *.ext.workplace.com