"A long-standing goal of the WordPress project is to be compatible with new versions of PHP on their release day. The next major version of PHP (version 8.0) is currently scheduled for release on November 26, 2020. WordPress Core contributors are working to ensure PHP 8.0 is supported in the next major version of WordPress (version 5.6), which is currently scheduled to be released on December 8, 2020."
https://make.wordpress.org/core/2020/10/06/call-for-testing-...
The WordPress ecosystem, on the other hand, will probably make the switch much sooner.
WordPress is the only web framework I've found that approaches stability of the Linux kernel.
Backwards compat is important and maintained.
Gutenberg hurt this a bit but it still does a great job of maintaining backwards compat.
I take old wordpress websites and update them and everything just works.
I take old laravel or code igniter or Drupal sites update them and everything just fucking falls over.
Fwiw, most modern WordPress development is JavaScript / React based. PHP is just a backend glue language at this point for things like templating.
Drupal 5/6/7/8 intentionally broke backward compatibility between versions in order to move the architecture and the feature set forward. The net result is that they are now at a point where Drupal could drop PHP 5 support. They also moved towards modern PHP / coding practices over the past years (Not invented here -> Proudly found elsewhere, composer, PSR's, OOP,...).
WordPress stuck with preserving backward compatibility and so the core API's have never seen a fundamental breaking change between versions. New features have been tacked onto what was already there over time.
I have kept a personal WordPress blog since 2005 and I have always been able to easily update things without migration pains. Of course, I run a minimum of customized code (beyond a custom theme that is). And I suppose YMMV and you still might have a different experience, if you did heavy customization through plugins, heavily leaning on WP's core API's.
Drupal? Totally different experience. Breaking changes and fundamental alterations of the database schema have been par for the course. I've seen 6 month long migration projects to migrate crucial enterprise data between versions. The PTSD of migrating from Drupal 6 to 7 to 8 is real.
Then again, WordPress and Drupal are different tools despite a significant use case overlap. I'd be weary to use either Drupal for a simple blog, or WordPress for an intricate, tailored back office application. I think they are both designed to do different things all together.
Having said that, it's worth noting that the Drupal project is very aware of past pains. One of the big goals of the past few years was working towards avoiding big breaking changes between major versions. It is purported that migrating between Drupal 8 and 9 is far less painful. Of course, it remains to be seen how that holds up in the long term.