If everyone refused to build guidance systems for missiles, perhaps the missiles would never get built?
I think likely yes, because if the zeitgeist among engineers was strong enough to make it nearly impossible to find one willing to build a missile guidance system, it would also be strong enough to make it impossible to find someone willing to build the actual missile.
To directly answer your question: no, I believe no weapon of war is OK. The reasons are all of the typical pacifist ones but I don't think it's necessary to reiterate verbatim here unless you feel otherwise.
> If everyone refused to build guidance systems for missiles, perhaps the missiles would never get built?
Correct but incomplete.
In that scenario, unguided rockets get built instead of missiles. Rockets, being unguided, will miss their targets more often than missiles. Rockets, being unguided, will also tend to be used in slightly-scattered volleys, so as to increase the chance that at least one rocket hits the target.
Any explosive ordnance that misses its target has some chance of instead killing some civilians.
This is the dumbest form of virtue signaling while erecting scare crows I've ever seen in an online forum.