The day that downshift protection got enabled, I remember being ticked off too. I thought it was too aggressive. Then I slowly tested to see what the redline speeds were for gears 2, 3 & 4 on certain cars and I would then go out and drive some laps aggressively. In watching the replays, and making a note at what my cars speed was when I was trying to engage 3rd or 2nd gear, it was shocking to notice how I was trying to downshift at speeds much too great. It was obviously a bad habit I had developed.
So, while it felt unrealistic...it was actually my driving that was unrealistic. Even with damage turned on, AC wasn't penalizing me enough for these horrendous downshifts in the name of "engine braking". I can't name every car that does or doesn't have a form of downshift protection in real life, but I think the way Assetto Corsa has implemented it is very good. It makes you use a little bit more of your brakes and I think that's actually more realistic. Just because you're used to being able to do something, in some cases for years of playing racing sims, doesn't mean that the new way isn't better. Yeah, it's going to feel unnatural and "wrong", but it's actually right.