Yes, there is a backported PHP 7.4 RPM for FreePBX 16 in the works, and it has gone through some QA but not nearly as full as usual, and if you’d like to help 
then please test on a v16 machine that you don’t mind potentially scrapping (mostly because the binary package overlap is a one-way trip right now when it comes to dependencies, and, it could break other things):
yum install sangoma-devel16
yum upgrade
reboot
EDIT: The fix landed in production less than a week after the above message, so there is no longer a need to install the devel/test repos. A yum update should be sufficient (then process restarts and/or reboot.) Also users of FreePBX versions <= v16 are strongly encouraged to update to v17 ASAP!