Drupal core is shipped with https://github.com/TYPO3/phar-stream-wrapper library. It should be updated in order to support PHP7.4
See: https://github.com/TYPO3/phar-stream-wrapper/issues/49
Seems like most of the compatibility issues are in tests, see https://github.com/TYPO3/phar-stream-wrapper/pull/50
Actual issue affecting the library has already been patched (but master branch only, which is PHP7+, and we need v2, which is PHP5+): https://github.com/TYPO3/phar-stream-wrapper/commit/27899b820ae69c500517...
