Hi Chris,
If you remove the licenses from those users, their users will remain, but all services will be disabled. Since they won't have an Exchange Online license assigned to them, they won't be able to exchange mails - they won't have a mailbox anymore. And finally after the 30-day grace period, all relevant data will be deleted.
In a nutshell, if you want your users to be able to have active email accounts, you shouldn't unassign their licenses, unless you have and are planning to assign a separate Exchange Online license or anything.
Regards,
Burak V.