Does Parallels Desktop support TRIM for Boot Camp based virtual machines?
Real TRIM is not supported for Boot Camp machines.
Guest TRIM is not acting with host SSD. TRIM is a low-level command, access to it restricted by filesystem driver in core of OS.
Technicaly speaking, from host OS perspective, virtual machine is just a simple application and do not has an access TRIM commands from host drive.
Please note that the main purpose of TRIM is to accelerate SSD performance.
Should you prefer to use TRIM, please import your Boot Camp partition into a Parallels Desktop virtual machine