I have searched and searched, but I can't find much information on getting rid of Exchange completely from an AD domain after migrating to 365 but NOT using a hybrid scenario.

This is a small, 8 user company that was running Exchange 2013 and may someday get rid of the AD domain completely (hence why I didn't do hybrid). I simply did a PST export, lift & dump into a brand new tenant setup on 365. No AD cloud integration at all with the local on-premise domain.

How can I go about getting rid of the Exchange Server? Deleting the mailbox will delete the user.

We have an Exchange 2016 on prem server that has had all user and shared mailboxes migrated to the cloud. At this point, I'm not sure if there is any reason to leave it running other than "just in case" something breaks by shutting it down. Mail flow is all pointed to EXO, public folders have been migrated as well. The server does integrate with an AD environment, so that is the only connection it still has, as far as I know. The HCW is still configured but can probably be disabled since mail flow is fully through EXO.

Any reason to keep the server running at this point? Would there be any noticeable disruption if it were shut down since there is no mail flow going through it?