I tested the following configuration on unstable which should work for stable, testing and future versions of php-fpm:
<FilesMatch ".+\.ph(ar|p|tml)$">
<If "-e '/run/php/php-fpm.sock'">
SetHandler "proxy:unix:/run/php/php-fpm.sock|fcgi://localhost"
</If>
<ElseIf "-e '/run/php/php7.4-fpm.sock'">
SetHandler "proxy:unix:/run/php/php7.4-fpm.sock|fcgi://localhost"
</ElseIf>
<ElseIf "-e '/run/php/php7.3-fpm.sock'">
SetHandler "proxy:unix:/run/php/php7.3-fpm.sock|fcgi://localhost"
</ElseIf>
<Else>
Require all denied
</Else>
</FilesMatch>
Since future versions of php-fpm will contain the update-alternatives setup for /run/php/php-fpm.sock, we don’t have handle future versions specifically anymore. I tested various combinations for socket names and symlinks in /run/php/ and it seems to work.