Multiple accounts across multiple instances.

Only if you want to play around weigh crypto bros.

All scripted: Our leaver’s manager gains access to their mailbox for 30 days, then access is revoked and the user account is unlicensed, and at day 60 the prem user account is deleted. The managers can copy any required emails into their mailbox or a shared mailbox in that 30 day period. There is no reason to keep every leaver mailbox around indefinitely.