Is it legally acceptable to advertise an apartment only for CH citizens or C permit holders?
I noticed that it may be common practice in Switzerland to discriminate against B and L holders, but is it legal?

Legal? I don't know, but if someone isn't going to rent to a non-C permit holder I'd rather know before going to look at the place and wasting my time.

Legal or not, if someone really doesn't want a non-Swiss citizen or non C-permit holder renting their apartment there not gonna rent to them regardless. F--d? Sure. Will it stop happening? Probably not.

Yes. It's completely up to the landlord to decide whom he wants to rent the apartment to. Sometimes you'll see ads where they'll only consider elderly couples or couples without kids or families with kids, etc.

They want to make sure you can stay and you are not here on a temporary basis. Some workers have a permit (A I think) where you can stay 9 months for seasonal work and leave for 3 months to your home country. But my understanding is that L and B permits are fine (EU and unlimited time migrants).