Mainly opening this issue as a reminder: Tuleap cannot be upgraded to PHP 7.4 until the following bug is fixed and released (probably in PHP 7.4.14):
https://bugs.php.net/bug.php?id=80458
It also means we will be forced to set a constraint "> 7.4.13" when the packages are updated to support PHP 7.4.
On a side note it is not the first time we get surprised by a change in a minor release. We should start testing the RCs, this one could have been bad on production instances.