My 8 year old does this once per week - he finishes school at 11:30am on that day, comes home on the bus and gets to the house around 11:45am and is home alone until his teenaged siblings get in (also via bus and own keys) at 1pm.
As you may have guessed from the timings we're abroad
It is completely normal here, we live in a village and he knows all the neighbors, it's completely expected and normal to look out for one another's kids, and the neighbors on either side are Sahm and offered to be his emergency contacts for school.
Primary school bags here have the key attachment built in and all my children had their own key attached inside their school bag "just in case" from their first day of school aged 6.
None of my children ever lost a key at primary school age, though dc1 managed to lose hers for the first time ever last month aged 14!
If they lose them we have a key safe hidden in the back garden which can be got to. I'm the only one who's ever used it when I locked myself out putting the bins out
When dc1 lost her key she was with dc2 who had his.
Dc3 used to go to an after school club until last summer, but was constantly stressed and tired being out 6:50am, when his bus stops in our village, to 5pm when I'd pick him up, and was having problems verging on being bullied at the after school club plus the staff were telling me he often "flipped out" during homework time and they'd send him out to play, so he came home with homework still unfinished. He struggled with the noise levels at the homework club. He's so much happier and doing so much better with his school and homework coming home straight after school, his grades have improved and even his handwriting has! He's just happier. Letting him come home has worked well and he's very much capable and alert, his self esteem is better too.
It depends on your set up, but people here - including teachers - are incredulous that in the UK we'd send 4 year olds to school but wouldn't let a 6 year old walk home without a parent or an 8 year old stay home alone... A lot of what we think are "obvious" minimum ages for things are just cultural norms.