As mentioned somewhere in this thread, using a custom domain poses other risks, in some cases more significant. All your aliases will be forever tied to your identity (and potentially de-anonymized by a single leak).
A separate domain can be used if really needed. But even with using my own domain, I don’t see it as a problem. After all, emails are not anonymous, and a leak with an alias with a custom domain is still meaningless and doesn’t affect other services.
So while it's tempting to use one random alias (h3hj4gjh234@yourdomain.com) for a high-risk service and another alias for a critical service (github@yourdomain.com), these aliases are easily identifiable as belonging to the same person.