PHP 7.4 is supported from Moodle 3.8.
The next version of PHP will be 8.0.
We should introduce a restricted PHP version check for PHP versions >= 7.5 into 3.8 and up.
We should really have done this when we removed the 7.4 restricted version check as we always knew that 7.4 was the benchmark, and not a higher version.
I feel it's better to make this change now, and not when 8.0 is out because some people will not update Moodle in the interim.