Anyone deemed clinically vulnerable.
As in, genuinely at risk. Were anyone with any degree of asthma deemed to be so? If so, then they have a decision to make.
As for the second point, it's self policed. By the person at risk. If that means their family are worried for their welfare, they make that decision also and are supported. Not the whole population stay indoors just in case, the at risk person self isolates, instead of absolutely everyone having to do it.
I don't think any country did do this, no. It was all out or none out. No doubt it is simpler to make everyone or no one do it, as the option of only those genuinely at risk or affected having to do it wasn't explored, but that doesn't make it right.
There is a far higher %age of the population, based on your own figure above, not at risk than at risk.
It's nice the way you're describing it but completely impractical. We're talking millions who would've been described as clinically vulnerable. That would've affected countless families with at least one member of the family classed as vulnerable.
And then what happens? You isolate the whole family or the individual? If it's the whole family, then the majority of households would likely have to lockdown anyway. If it's the individual alone it would never work which is why whole households were told to isolate when one member tested positive.
And if we allowed ppl to decide themselves and police themselves you'd have a free for all with ppl being off work anyway.
Add to that, if you had a person in your household who was vulnerable, you and every other member of the family could be as responsible as you want, go out and you'd be relying on everyone else out there being responsible too which as we know they weren't.
Let's not forget during the lockdown most on here were criticising the general public for not using their common sense, I remember that very well as the government were getting lambasted for the idiocy of many.
The level of administration and oversight in trying to deal with the complexity of 1, 2 or any number of household members deciding for themselves randomly whether to isolate, businesses having to deal with it, and then claim furlough, as well as deal with staffing and everything associated with it would've been insane.
I dare say all of that is why no country entertained the idea. It would've been a mess and tbh pointless.
I know in theory it's a nice, neat way of dealing with it but in reality it's not how it works. It isn't like all the vulnerable ppl lived together and all the fit and healthy folk lived separately with no interaction.