Once upon a time, we could see recently used (90 days back) display names, then according to this, to know when we can change again, in case 30 days didn't yet pass since last used name.
However, this feature was removed.
Instead of asking to bring back the past names, I suggest something different: in case we can't change the display name on a site, have the field disabled, with a lock icon, and show in a tooltip when we can change again:
Can this please be done? Currently the only way to know is to try changing the name, submit, then see the error message. Not very elegant.