Hello Chris,
honestly, your question surprises me a bit, because for me it's absloutely obvious that this function has always been included.
However, as you wrote, yes it is an operation that I have always been able to do when creating Custom Basemaps in every version of ArcGIS Pro until now (3.3.1).
And this is a Custom Basemap with a Thumbnail created with ArCGIS Pro 3.2.2, last version without this bug.
Just to be clear, this with the same service. You can try for yourself with this REST service from Regione Lombardia: BaseMap/Ortofoto2021 (ImageServer) (servizirl.it)
Cheers
Stefano