Exchange Online Shared Mailboxes deleted after deleting disabled users from local AD

Here is the support article from Microsoft outlining what you were trying to do (with the warning not to delete the user) as well as the steps to recover if you did delete the user - https://support.office.com/en-us/article/Convert-a-user-mailbox-to-a-shared-mailbox-2e122487-e1f5-4f26-ba41-5689249d93ba

I'm also not aware of it being a license violation based on everything I've seen on the topic.  I know all the users that access the shared mailbox must have a license, but not aware of any  violations of using a shared mailbox to preserve a mailbox, especially if you need to continue receiving email to said mailbox.  Here is another good article on the topic as well https://practical365.com/exchange-online/shared-mailboxes-vs-inactive-mailboxes-departed-users/.  Still doing some looking into the licensing issue, so I'll update the thread as well if I can find any more details around it.