Latest news on that topic is, that it's most likely coming by the end of the year, when Plesk 17.9 is out.
As of the 27th of February 2019, Postfix does now support SNI (see
Postfix stable release 3.4.0), so the base for supporting this feature on Linux is finally here.
But, you still have to:
- wait till this new Postfix version is shipped with your chosen OS* for Plesk
- or hope that Plesk will build their own Postfix packages and ship for your OS
* So far I've only checked Debian, and here it looks quite good to see Postfix 3.4 getting included into version 10/buster, that is supposed to be releases this summer - so maybe early enough to be also officially supported by Plesk when v17.9 ships