PHP 7.4 has nearly a year left of security fixes before it is end of life.
Probably best to spend that time updating your code for version 8. Then make the switch when you are ready. https://www.php.net/supported-versions.php
Many web hosts allow you to specify a PHP version per directory. For instance I know Siteground does this and probably bluehost as well. Usually in the control panel of your host you will see something like “PHP version manager”. In there they let you select the directory and then the version of PHP to run all scripts in that directory as.
Now having said that, as others have stated, you might want to look at finding a replacement plugin if the developer doesn’t want to upgrade their script. That or wait. While some hosts will keep legacy versions around for a bit longer (even if no more security updates are made by the PHP community) it is good to make sure you can keep up on the version out of best practice and for optimal security.
Docker would be the simplest solution if you have shell access to the host. Roll up two php-fpm containers with different PHP versions, mount site folders as volumes and add single nginx container to route requests for each site. They can share the database as well.
More than that, it is even possible to run different parts of the same site with different PHP versions, if URLs of these parts can be strictly distincted. In that case you just add multiple Location blocks to the nginx config.
I can make a sample docker-compose.yml file, if you’re interested.