This used to be included back in the 2015 versions and was taken out for who knows what reason. Now in order to flatten I have to go back to a 2015 version temporarily or use the online workaround SLDPRT file.
Either way, its doable, so just make it a Professional Feature. Features should all be included in Professional - Premium should be saved for more complex upgrades. This just makes little sense and is a nuisance that they took it out artificially.