You will probably have problems as yet with that version of PHP, however there is no reason that your system cannot have more than one version of php installed, just install php 8.0.5 (the current latest available) and likely just simply change any #! lines to refer to that version.
You might have challenges with commercial modules since the module licensing infrastructure in FreePBX 15 runs only on php 5.x, but I think most of the non commercial freepbx modules work with at least php 7.x.
For FreePBX 16 commercial and open source modules should be ok though running on php 7.x.
I’d back the system up or at least make sure you can revert it if it’s important though before trying things out by hand though.