Just commenting here with my test results for reference of other users.
I was able to complete further testing on this, and I am reaching out to outline my findings. First, I configured one of my test servers so that only TLS 1.0 was supported, then configured another test server (which is configured for TLS 1.2+ only) to relay mail to this test server for a particular domain. What I found during the SMTP session is that the initial attempt to deliver uses STARTTLS, fails, then the second attempt ignores the STARTTLS flag and transmits in the clear.
So this looks to be working as expected in my own testing at least! With that out of the way though, disabling TLS 1.0/1.1 will definitely lead to connectivity issues on older email clients and devices, so this is something you'll want to roll out with plenty of prep beforehand. Hope this helps!
Kyle Kerst
System/Network Administrator
SmarterTools Inc.
(877) 357-6278
www.smartertools.com