WordPress in the currently latest version 5.5.3 produces a 404 error when trying to enable just released PHP 8. Why?

Officially WordPress will only be compatible with PHP 8 from version 5.6, which is scheduled for December 8, 2020. RC core version of WordPress 5.6 works correctly with PHP 8, the problem has been fixed. However, it is interesting to understand what is the source of the problem.