As noted here on the SimpleMDM blog, an option was added to iOS starting in 14.x where individual apps can be marked as "non-removable" by the administrator when being deployed.

https://simplemdm.com/blog/mdm-ios-14-macos-11-big-sur/
https://it-training.apple.com/tutorials/deployment/dm195

"To prevent a user from uninstalling a managed app, mark the app as nonremovable when you assign the managed app to a user or device. Depending on your MDM solution, you might need to deselect a Removable attribute or set a nonremovable attribute. With your MDM solution, you may also be able to set this attribute on apps that are already installed on a device."

I feel it would be a valuable feature to implement this flag so that users can still have the ability to add and remove their own apps on their managed device (which would otherwise not be possible via a restriction profile block), but are prevented from removing apps deployed by policy (Such as security apps, or authentication apps required by the company)