I tried using Linkedin ads to promote about 6 of my mid-range domains, I saw an offer from Linkedin a while back for £100 free credit, spent £50 of my own money, paused my ads and then Linkedin offered me another £75 free credit. Didn't convert into even a single offer (the domains were priced but with the option to make an offer).
I do still think there is some potential here for others to try, I wouldn't totally count it out. I'd recommend everyone at least find one of the free credit offers to do a trial.