This isn't a bug per se, so I'm not sure I should be using Phabricator, but - if you go to the main site for OOUI demos:
...and then click on the "PHP" button, you get to here:
Which displays the following error message:
Composer detected issues in your platform: Your Composer dependencies require a PHP version ">= 7.2.9".
There are a few ways to fix this, but I'm sure whoever is reading this knows these as well as I do.