TT now supports the efiling of amended returns however there are a couple of exceptions like you cannot change filing statuses and you had to have efiled the original return and the original return must be fully processed ... plus a couple other rules. 

Yes you can now efile a 2019 amended return from Turbo Tax.  But not prior years.  

@Critter-3 , @VolvoGirl  Thanks for prompt replies! Has TurboTax documented the rules for e-filing amendments? Couldn't find any clarifications from TurboTax on this...

Only FAQ I found

https://ttlc.intuit.com/community/amending/help/can-i-e-file-my-1040x-to-amend-my-return/00/26008 

@VolvoGirl   link is broken

hmmmm,  wonder why that happens sometimes.  It doesn't like pasting.  I'll try another way

https://ttlc.intuit.com/community/amending/help/can-i-e-file-my-1040x-to-amend-my-return/00/26008 

Nothing in official writing has been posted by TT yet however my professional program posted some limitations ... more to come from the IRS as the kinks are ironed out ...  https://www.irs.gov/forms-pubs/about-form-1040x