I would be very sad to see this RFC not go through. I have voted Yes as I believe this "feature":is a bug that needs to be fixed. There is also an opt-in proposed for people who really do consider it a feature.
I don't see why it would cause much trouble for maintainers of OSS. At the moment it is proposed to make this change in PHP 9.0, which is a couple years away. That is a lot of time to fix the code. The deprecation message will inform us about the number of uses, whether accidental or intentional. If we decide that removal of this feature would cause too much disruption, could we not have RFC in PHP 8.3 to remove the deprecation? Deprecated or not, I still believe that OSS should avoid dynamic properties. They are really difficult to identify, even with static analysers. Having the deprecation message would at least help us identify where dynamic properties were used, so that we can fix it.
