Currently CentralAuth allow stewards to delete global accounts. It is a rarely used feature: https://meta.wikimedia.org/wiki/Special:Log?type=globalauth&user=&page=&wpdate=&tagfilter=&subtype=delete&wpFormIdentifier=logeventslist

If the global account is not empty, the process will copy password to local wikis and leave local accounts unattached. However (1) local password is something we should phase out (cf T104500) and (2) unattached local users does not work well with SUL3 (since we will handle password change in loginwiki only) and it is proposed to treat all local users as attached (T367968).

If there are any use case to delete a global account, it can be done manually server-side.