Is fasting lunch instead of breakfast, a good idea?

If it were me, I'd probably forget about fasting..unless I were simply not hungry from breakfast to dinner. A salad or light sandwich would probably be healthier than skipping lunch, I'd guess.

With that choice I would definitely skip the lunch time, so your good to go imo. There's no storage back up reserve for protein and considering we've been sleeping for a few hours, up to 8 or 10 then skipping breakfast and eating basically 12 or so hours between meals and if we not only want to maximize muscle protein synthesis, we also need protein in a reasonable timeframe to ensure all the other metabolic processes where protein is required which are many, then not having a decent amount of protein from our diet ready for absorption the body will just take it from muscle, bone and other areas. And in that regard getting protrein at lunch time in say a protein shake would be ideal, basically what I'm saying is 3 meals is better than 2 in this contxt. Not only that, we need leucine to be that driver, so that is also important. I've recently changed my mind on meal timing and getting enough protein, spread properly throughout the day is imperative for not only to protect our muscle mass, bone etc. it's even more crucial as we age. imo

it may help if you are trying to control your diet by having smaller eating windows but from the research I have done on intermittent fasting you need a minimum no eat window of 12 hours to benefit from intermittent fasting. I actually focus solely on my fast window so the earlier I am done eating at night the sooner I can eat the following day.

as long as you're hitting your deficit it really comes down to whatever is easier for you to adhere to.