Knowing you have a stealth ICBM doesn't change a thing if there is no way for the other side to defend against the non-stealth ones, so it wouldn't be a deterrent to most possible adversaries, but the optics of disclosing such a thing developed in, say, the 90s wouldn't be great.

Those that might conceivably try to defend themselves against conventional ICBMs are few and might very well know such a thing exists, it may work as a deterrent in that context.

If such a thing exists at all, not quite sure how you might make an ICBM stealthy, couldn't think of a way to hide launch and reentry signatures...