We have several places in the framework checking for existence of arbitrary class names, allowing classes, interfaces or traits (see #40140 for instance which adds such a usage, but I'm almost sure we have others elsewhere).
As PHP 8.1 will introduce the support of enums (the RFC has been accepted), these places should be updated to also check enum_exists

symfony/polyfill#335 might help here to avoid checking for the availability of enum_exists|