Of course, the MySQL extensions have been retired from PHP for a reason and should no longer be used. Instead the equivalent more secure MySQLi should be preferred. This said, of course you know this and you know you had to update your (old) scripts but there will always be situations in which you just needed to upgrade your PHP version and just never quite got around to updating all scripts. And you'll find yourself, again, with unsupported Mysql calls and functions.
Credit where credit is due, the info can be found on https://ckon.wordpress.com but then I thought; one can compile it themselves if really needed (which is always the best solution) but there will be cases compiling fails and rather than debugging why it fails, wouldn't it be handy to just already have the mysql.so libraries compiled available for PHP ?
These were compiled on a Linux Debian 10 amd64 with PHP 7.3 and PHP 7.4. There is no garantee they will work on any system but if you're in a real hurry, you may want to try these and just add or modify this in your php.ini file :
And be sure to get on it now and start updating the (old) scripts still using mysql.so ;)