For declared classes I think it's a good idea.
We have legacy code that is littered with dynamic properties using
stdClass. It runs today on PHP 7.4.22 and we plan to migrate to PHP 8
as soon as some Open source libraries works under PHP 8.
Being forced to rewrite this if this proposal changes to also disallow
stdClass dynamic properties would have zero benefits for the app itself
and only incur a cost.
Regards //BjÃ¶rn L