Maybe need allow the short tag for v8?
Version 8 system requirements PHP 5.5.9+ (PHP 5.6 or 7 recommended.).
Short tag "<?=" always available since PHP 5.4.0http://php.net/manual/en/migration54.new-features.php...
Why this tag forbidden in packages?
Hence the one size fits all approach. We all compromise to a median (Including PRB members and Portland Labs), which is the current guidelines.
That's not a comment on this specific issue, but a generalisation that makes the whole system workable for the amount of resources available.
Currently I don't think we can justify unnecessarily diverging guidelines for 5.7 and v8.
For short tags, amongst other ways they can be automatically expanded using the c5 command line. Work in short tags as much as you like, then replace them as part of your final build process. That is what the core does.
When the c5 core is released with short tags and core support for non short-tag versions of php is completely dropped, that will be when the overall guideline can change.
In the example you pasted, there is a space between the question mark and the equals sign.
I wonder if the marketplace linter is seeing that as "<?". If so, it is seeing it as a short open tag and not echo shortcut syntax.
But to keep compatibility high you should never use short tags in 3rd party items as you cannot guarantee the user buying your code has them enabled.
This directive also affected the shorthand <?= before PHP 5.4.0, which is identical to <? echo. Use of this shortcut required short_open_tag to be on. Since PHP 5.4.0, <?= is always available.
The marketplace linter also lints 5.6 and 5.7 add-ons and themes. The PHP minimum for those is below 5.4.
As always, things are moving forward, and I think that more and more people will switch to v8.
I don't think it'll be a problem for PRB reviewers, since it should be an automatic test that doesn't need manual review.
Imposing to not use short echo tags would be a kind of "violence" against package developers. With no valid reason imho.