No, custom functionality testing require lots of hours of testing in order to fulfill client satisfaction. It does not cover under our website maintenance services plan.
Having a custom functionality on a WordPress site might not be compatible to future updates such as the WordPress Theme newer version which may cause another issue when upgraded to new theme version or even WordPress latest version. It is best to test it on a staging server instead of doing it on the production server. Always perform a backup just in case it causes the whole website to be downtime. Backup can assist to restore back to original before performing custom functionality testing.