You used to be able to do this, but e-mail has been required for some time.
Over on Lemmy, it is the instance operator's choice whether or not to require e-mail, but most instances currently do as a measure to mitigate bot signups.
>>Sprock+er
Yeah, unfortunately, it only works on old reddit. It's quite amusing to see how they treat old and new reddit as entirely different platforms.
>>johnny+Oj1
This was also my experience when I tried to register a custom domain. The only thing that worked for me was using a Gmail account that I had used with other, older, accounts.