LTR (Long Term Retention) backup for Managed Instance is being worked on and should be expected in a preview in calendar year 2020. Updates will follow when the feature becomes available.
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The fact that there are no snapshots and no long term backup/PITR retention with managed instance leaves folks who are using TDE with no real option to pick an "older" point-in-time view of their database for audit/compliance/testing/debugging purposes. I have to imagine lots of folks are going to have a problem with that. And as Rob said below, turning off TDE to take a backup and then turning it back on is hardly an elegant (or compliant) solution.
My vote would be to just have a more sticky retention for the automatic backups, with the option to specify particular backups for "indefinite" storage. I might have a backup (lets say like the last day of the year) that I want to keep basically forever, but I don't need the daily's for every day in-between. Or give us access to the storage account where the backups live and the ability to manage that process on our own.