The assets in your Apple Business Manager that are listed as Released cannot be deleted. You opted to release the assets (lease return, sale, donation, damage, etc.). However, the presence of the prior ownership remains. You will not be able to removed released assets.
The units labeled Unassigned simply means that you have not assigned those assets to an MDM. This leads to the question of, "is this intentional to allow use of the device outside of management, or is this a mistake and these devices should have been released from chain of custody?" Perform an inventory review to determine which devices are still viable. If you have units listed as Unassigned that have been removed from production, you likely should release the assets to disassociate the hardware from your organization.
Finally, the title suggests that you are trying to migrate from one MDM (Apple Business Essentials - which is really not great) to another (Intune - which also has a number of frustrating limitations). If this is true, you might want to get all your devices up to the latest OS version (iOS 26.x, iPadOS 26.x, macOS 26.x, etc.) and then use the new migration feature in ABM. This allows the (relatively) seamless move of production devices from one MDM to another with limited user interactivity. And it also means you can do it one device at a time so you can refine the process through iteration.
Hope this is helpful. Good luck on your project.