Topical steroid and Vit D ointments, creams and foams DO suppress immune responses at a cellular level - that's how they work. Obviously the effect is more localised than if the patient is injecting, say, a biologic like Adalimumab.
Take Enstilar foam, for example. Here's part of an official description:
"In addition, vitamin D has an immunomodulatory effect, suppressing activation and differentiation of Th17/Th1 cells while inducing a Th2/Treg response. In psoriasis, corticosteroids suppress the immune system, particularly pro-inflammatory cytokines and chemokines, thereby inhibiting T-cell activation."
Psoriasis is not just a skin rash. It's an autoimmune disease in its own right, and is frequently co-morbid with other autoimmune conditions. The treatments for moderate to severe psoriasis are designed to interfere with the immune irregularities, and these can then bring their own risks, e.g. increased susceptibility to infections.
It's unknown territory, throwing Covid-19 into the mix.
I've had psoriasis since I was 13 and it's a bastard.