I have, though I admit it's been about 10 years since and my memory may be mixing in (pun intended) things from version 1.
Apparently there's now something called annotation routes. Which appear to be inferring functionality from comments.
You made me look though. I suppose I had it coming.
[0] https://www.php.net/manual/en/language.attributes.overview.p...
Unknown attributes are also ignored silently, which isn't really a good sign.
It may technically be forwards compatible syntactically (`#[foo]` will, as you say, just be ignored by PHP 7), but that's an anti-feature (assuming that the attribute isn't a no-op, it'll presumably break something else).
> `#[foo` used to be legal PHP 7, but is illegal in PHP 8).
So it is not a comment in PHP8 then.