I think it totally depends on the child and the context - as in how sensible they are and if the home environment/community is generally safe.
Fwiw, when I was 12, I was home every single day of the holidays by myself, I assume those would have been 12+ hour days. My mum had a 1.5 hour commute, so I'm guessing she would have left at 6/7am and been home 6/7pm every day. It was the 90s, so no working remotely and no mobile phones. We had no family who could have helped and while there were activities for a 12 year old, they were properly expensive ones (not like holiday club for a 5 year old), like pony club camp for a week and none that would have had me from so early to so late. So staying home was the only option.
It was absolutely fine! I watched tv, went for walks/bike rides, played with the dog, read books, etc. There was no internet back then and I didn't really have friends within walking distance, though I did sometimes walk (say, 30-60 minutes) to go visit friends. Around that time, I was doing overnights as well, 3-4 nights at a time, I'd take myself to school and home, look after the dog, cook my meals, do my washing. It was great for independence, but looking back now, seems terrifying that it was the pre-mobile phone era. I leave my 11 year old home alone for an hour or so here and there, but it would feel very different if I had no way of contacting her.