I noticed the 'deprecated' message at top of this cmdlet Invoke-SPOMigrationEncryptUploadSubmit with a link directing to SPMT PowerShell cmdlets. This makes me wonder if Microsoft is replacing the migration APIs with SPMT migration engine.
Could it be correct? Would this mean the migration tools will be using SMPT engine under the hood for migrations than the migration APIs? E.g., I believe Sharegate use the migration API for insane mode.