Allow devices to be marked as unmanaged
Another idea to borrow from Jamf Pro. Please consider allowing SimpleMDM customers to mark devices as unmanaged in order to keep them for historical or data purposes without them actively being under management and NOT taking up a license. An unmanaged device would keep the same config, but not have the ability to interact with MDM moving forward. No profile installs, polling for device info, etc. It is a record stuck in time. Devices should be able to move freely between managed/unmanaged through the GUI and API.
As it today, every device which exists in a SimpleMDM tenant takes up a license. No matter how long ago the device was enrolled, when it last checked in, etc. a license is still consumed. Though I understand the advantage on the PDQ side for billing and revenue purposes, it would be useful as a customer to be able to keep devices within SimpleMDM as unmanaged without having to pay for the corresponding license. As best I can tell, when a device in removed it's data is not also removed. I've tested this by removing a device and re-enrolling it. Immediately after re-enrollment the old data is still present, even before MDM has had a chance to report back. My theory is when a device is removed it's simply tagged/marked as removed, with all other associated data remaining in the database. If that's the case, it's a zero or neglible cost to allow customers to mark devices as unmanaged since the only difference is whether the device is visible and requesting regular MDM actions.
The only other options are for customers to buy more licenses (probably what PDQ prefers) or run regular cleanup to remove old/stale/retired/archived devices. While MDM is not and should not be an inventory, there are common operational and compliance needs to keep MDM device records around for longer even for devices which aren't being managed. So tl;dr - Allow devices to be marked as managed and unmanaged.