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).
![SMauri_2-1720615856996.png SMauri_2-1720615856996.png](https://community.esri.com/t5/image/serverpage/image-id/109300i0F26E21EE2A7C939/image-size/medium?v=v2&px=400)
And this is a Custom Basemap with a Thumbnail created with ArCGIS Pro 3.2.2, last version without this bug.
![SMauri_3-1720615936473.png SMauri_3-1720615936473.png](https://community.esri.com/t5/image/serverpage/image-id/109301iA56611ED58FEB401/image-size/medium?v=v2&px=400)
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