Since PHP 7.4 has a release candidate, it can be considered nearly stable. Now we can start thinking about the compatibility.
The following is the problems from phpunit unit test:
Function ReflectionType::__toString() is deprecated tests/phpunit/unit/includes/watcheditem/NoWriteWatchedItemStoreUnitTest.php:225 tests/phpunit/MediaWikiUnitTestCase.php:110 maintenance/doMaintenance.php:99
- Cannot use "parent" when current class scope has no parent (php-7.4.0RC1 note)
- Deprecated: Array and string offset access syntax with curly braces is deprecated in avro-php lib. (php-7.4.0RC1 note)