It's holding PHP back in the sense that because there's so many WordPress installations out there running on old versions of PHP and old versions of WordPress, that _will_ have to update at some point, it means that PHP internal devs need to always consider how any breaking change they make would affect those users.

In general, the WordPress community is _slow_ and plugin authors and users definitely do not keep up at the same pace at the rest of the community using more modern frameworks. They tend to have written worse code in general, which are proportionately more impacted by deprecations and error promotions where the code wasn't using best practices.

The PHP internals maintainers hear complaints from people in the WordPress space saying "whoa you're making too many breaking changes too fast, I can't keep up", meanwhile having the developers of modern frameworks getting impatient, wanting all the new features to save them time in writing new code.

If PHP didn't have all these legacy projects still in use, deprecations/strictness changes could move at a faster pace.