Recommend Steps for Deleting Microsoft 365 User

1. Confirm OneDrive retention is right for the company at default its 30 days
   https://docs.microsoft.com/en-us/onedrive/set-retention
2. Decide who is getting access to the users OneDrive and emails
3. Depending on if the account is AD synced or Cloud only do one of the following
1. AD Sync
1. Delete user in AD and force a delta
2. Confirm user has been related out of Microsoft 365
3. Restore user in 365 and reassign its license.
   
   It will now be a cloud only account with no sync back to On-Prem, form here can follow cloud only
2. Cloud Only
1. Select user in Active users and delete (Follow recommend flow)
2. Give access to users OneDrive as needed
3. Give Access to users Emails as needed
   
   If you are doing this from a recently broken synced account it may take a few minutes and a page refresh for the right options to appear in delete options.
   https://docs.microsoft.com/en-us/microsoft-365/admin/add-users/delete-a-user?view=o365-worldwide 

This is a secure and clean way to handle user deletion while keeping on-prem AD clean while also fixing an issue with hybrid exchange set ups / migrations which can result in shared mailboxes getting deleted from Microsoft 365 if they once where a private mailbox moved from on-prem exchange 

Convert a user mailbox to a shared mailbox

https://docs.microsoft.com/en-us/microsoft-365/admin/email/convert-user-mailbox-to-shared-mailbox?view=o365-worldwide

Side note

Quick consideration when converting to a shared mailbox,  this can be done to free up the license as long as the mailbox is under 50GB, any larger and it will still need a license.