[Nullabillity]

It isn't backwards compatible (`#[foo` used to be legal PHP 7, but is illegal in PHP 8).

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).

[tored]

Okay, forward compatible. It makes it easier for framework and library maintainers to support multiple versions of PHP, which is nice.

> `#[foo` used to be legal PHP 7, but is illegal in PHP 8).

So it is not a comment in PHP8 then.