Apple Business Manager and Apple Developer Program
No - there are multiple avenues for distribution. MDM can assist with many and it depends on the MDM and the app entitlements and where and how you upload the app.
When a developer account expires, you lose control and can’t renew signing certificates. Currently, Apple doesn’t pull these apps, but there’s  no reason they couldn’t in that case from my reading of our contracts in the US.

With B2B custom store, public store with private SKU pricing and more, you have diverse options here.

https://www.apple.com/legal/enterprise/apple-business-manager/abm-us.pdf