There are several safety PLCs that support safety communications links.
Beckhoff has TwinSafe that runs over EtherCat.
AB, simens, etc all have theres. I have not heard of any safety over Modbus safety controllers though.
I am in no way an expert or even a novice on this subject, but I believe that the one thing that all "safety comms" have in common is that they are built on deterministic protocols.
The RS485 spec is pretty old and the com speed is very slow compared to modern communications, so I wouldn't really want to use it for safety.