I've always just assume everyone I've played with assumed their characters were accruing minor wounds as their characters hp went down and were roughly aware of that status.
That is certainly one way to look at HP and perfectly acceptable.
But because HP represents
so many abstract things, do any of the losses have to be minor wounds at all? That 5 damage attack was "dodged" or "parried" maybe and expended HP are energy to avoid it so it wasn't actually a serious hit, because otherwise those 5 hp
could KILL (compared to the commoner with 4 hp total...). Such "energy" can be quickly recovered by resting (short or long), or restored by magic (healing) so the PC feels revitalized. It would be easier to argue recovering such hit points instead of those "minor wounds" healing overnight...
I can see the value in a system that gives gradually increasing penalties as you go.
Such a system could have value, but then argues is the granularity worth the bookkeeping.
For as much simplicity as I could suggest the following if I continued with D&D:
OPTION 1:
A straight -1 penalty for each 20% reduction in hp. Yes, this requires a bit of bookkeeping, but not too much IMO. You list your maximum (100%) hp, 80%, 60%, 40%, and 20%. If you are between 81-100%, no problems. At 61-80% you have a -1, at 41-60% a -2, and so on.
OPTION 2:
Have a damage threshold equal to 20% hp. Each time you lose that many hp, you gain a level of exhaustion.
It seems strange to me to insist that "hit for five" doesn't give the character any sense at all of their danger/safety based on their current HP unless the DM narrates if it was a scratch or a cut or a bruise or a parry, or let's them know if they're starting to feel winded. It seems kind of akin to a DM making a character act out their social interactions instead of just saying their impressive looking barbarian tried to intimidate the clerk.
Well,
someone needs to narrate it, either the DM or the player.

Otherwise, you just don't know other than the numbers...
I narrate a lot depending on the damage and hp of the target. A hit for 5-10% might be a parry by shield, but the PC "feels" the impact and strain on the arm and shoulder from taking the blow. A hit for 20% might be a cut to the side, painful and perhaps links of armor are rended, etc.