The forum will continue to work in its current state. You simply lose access to all IPS services, including updates, the IPS spam service, support etc.
I would point out, it would be worth ensuring the customisations are updated, and if needed move to a server that supports PHP 8. By running PHP 7, you are actually running a site that could potentially be compromised, as PHP 7 is no longer supported by PHP themselves for security fixes.