Gota bee in your bonnet Flower?
I suspect the children on whom you are basing your personal views of how old a child can sensibly be left alone for 20 mins are not nearly as mature and sensible for their ages as you think (I.e. far less so than the children whose parents do leave them alone for that length of time) or its all about you - your paranoia, your need to be deep, deep in your comfort zone all the time, your fear of being judged, your mixed up Iidea that the more over protective you are the better a parent you are - or your unwillingness to teach your children how to deal with all reasonably likely scenarios and drill them in emergency procedures until they are second nature.
As others have said it takes far more effort to bring a child to the stage where they have the skills and confidence to be left than just to keep them at your elbow all day.
As others have said too, the role of a parent isn't primarily to be Iin the same room/ building as a child at all times, it is to build slowly, slowly, towards that child's self reliance and independence.
My older children could do everything on that NSPCC list from age 7 - yes, reliably and confidently. I know they get out on autopilot if the fire alarm beeps because it'shappened, I know they phone my mobile Iif they have a query when I'mpicking their little brother up (or taking one of them to a friend/ club etc.) If the children you know couldn't reliably manage they shouldn't be left, but fortunately you don't get to make decisions for anyone else, and your blanket declarations are nonsense based on nobody knows what, but ccertainly not wide knowledge of the capabilities of my kids or immediate neighborhood and support network nor those of the other posters who have risk assessed their own situations.
I make no secret of leaving my 7.5 year old and/ or 9 yo - the 4 year old ' s Kindergarten staff know and agree it's aappropriate at this stage, school staff know and expect it as it's utterly normal, neighbors and fellow school mums know and do the same - I double checked with another mum a few weeks ago whether she was ok with me leaving her 9 yo home with my 9 yo while I fetched my 7 yo during a play date or whether I should take her and she laughed aand said of course I shouldleave them home - tthey're 9!
I sometimes get frustrated living Iin a country not of my birth and having to operate in my 2nd language, but thank fuck Iin Germany no pearl clutching hand wringer thinks the police or social services will do anythingbut laugh Iif they rang to "report" me for leaving a 9 year old homealone for 20-30 mins :o