Not all aircraft were designed with, or are required to have, a stall warning system. However, I would suspect that those which were never fitted with that system are "older" types, as I think that just about all "newer" certified aircraft have a stall warning system. I have no idea about the Robin.
If the aircraft has a stall warning system, that system was certainly used as a part of the showing of design compliance for the certification of the aircraft. Therefore flying with the system inoperative would be a violation of the basis of certification, and require a design approval (STC) to permit that operation.
A flashing beakon, on the other hand, might be required by operational rules, but is not required for a showing of design compliance (for the basic design in DAY flight), and therefore does not need to be fitted, or if fitted, operational, unless an operational rule says so, because the design does not. If the aircraft manufacturer required that a beakon be operational on the aircraft, it would likely be stated in the Type Certificate Data Sheet, as are many other fixed requirements.
Safe and legal. Safe is pretty clear. Legal will come from a number of different sides, and you have to cover them all.
Pilot DAR