So this the official reply from WordPress Core team regarding short-echo-tag “<?=” - saying it is allowed. As mentioned - the “Envato WordPress Plugin Requirements Page” has to be fixed:
By @dd32 (Dion Hulse)
I’m closing this ticket for reasons previously outlined, and there being no Meta tasks needed here.
In the event the Core team decides to change the coding standards for WordPress Core they can update the Core Handbook.
Plugin authors can use short tags if they wish already, they’re not held to the same PHPCS rules of Core unless they choose to , they may use whatever MVC framework they want, and can require the usage of PHP 7.3 if they so desire.
Replies can still be added to this ticket if needed, and if required the ticket will be re-opened by a meta team member, but please consider adding a comment outlining why a meta ticket is needed prior to doing so.