Not sure if I can believe you that such a failsafe exists, but it's pretty stupid if it was implemented like that with nobody in the game telling you about it. So if I've lost half a heart, I am subject to instant kills, but with half a heart more I'd somehow survive an attack that would kill me three times over.

I find it very strange that something that is supposedly as consistent as you say has never triggered for me at any point. Because even though I tend to play with the attitude of not healing myself until I've lost half of my hearts, shrines do replenish hearts completely, as does sleeping at the base camp near Hyrule Castle which I frequently did when I was down to one heart after a completed battle with no new shrine in sight. So there have been lots of occasions where I've been running around at full health.

There's a side quest that lights up all the cave entrances with a huge pillar of light, visible from anywhere in the area.

Wait, what

I definitely missed this quest 

Dunno if I'd call it a quest but if

Dunno if it's location or enemy type dependent, but there has been one occasion where I had 4 hearts, full health, got hit by a lynel and had a quarter heart left. It struck me as odd at the time and apart from that instance I never walk around with full health.

Anyway it wouldn't surprise me, there are many weird mechanics in the game and this one (if true) further cements the idea that there is no point in upgrading total hearts. That would just require better food to heal back to full after a hit. Stick with 4 hearts and all you need is lots of 3.75 heart meals! But it wouldn't surprise me either if it was not consistent and entirely depends on the enemy that hits you.

There are 2 health bars btw. I always walk around with 1 heart for desperate strength double damage, yet the light roots restore my gloomed out hearts. A lot of the enemies down in the depths take out the gloom free black hearts first, thus leaving my 1 red heart intact. No idea whether a gloom based attack also has a fail safe when still being gloom free. (But not full health)

Is there a way to reduce total hearts, if I could have just one then permanent desperate strength bonus and with the fail safe, just eat a shroom after each hit lol.

I doubt that gloom-based attacks have a failsafe, because they deal exclusively full hearts of damage.

There's a way to reduce hearts, but only down to 3 hearts and not below. Just like in BotW, there's a demon statue that takes earned containers from you and then allows you to reapply such a container as one of your choice (heart or stamina). That would be the only way for me now to verify that weird failsafe mechanic for non-gloom attacks, because I now have full stamina and 36 hearts after completing 149 shrines.

I am at 57% map completion now, so after doing the shrines I can alternate between looking for caves and wells, collecting korok seeds and doing sidequests. Looking at the post above, I should do sidequests first. Also managed to complete all main objectives today, because on Monday when I beat the game I hadn't done the search for the final sage yet.

Ok, it's not a quest indeed. I was right next a cherry blossom tree and it worked. Thanks for sharing

This mechanic definitely exists and has existed since Breath of the Wild. This only marks the second game in a row where they've kept it hidden from players. I'm not entirely sure how exactly it activates, it doesn't always save you from a one-shot at full health, I think it's based on how much damage you take.

For reference I just booted up Breath of the Wild and tested it with the Decayed Guardian outside the Bomb shrine. Its laser deals 14.5 hearts of damage if you wear nothing. So I got rid of a few hearts...

At full health with 10 hearts, I took the hit and survived with a quarter heart.
But with 9.75 hearts, it one-shot me.
And now at 9 hearts, full health, it also one-shot me.

Tested all of these multiple times so it's not RNG shenanigans, I think it's protecting me from damage up to 50% more than the amount of hearts I have. Not sure if it works exactly the same in Tears of the Kingdom but I would assume so, they kept most little mechanics like this unchanged.

Interesting and worth keeping in mind for the next Zelda game (or master mode in TotK if it gets added down the line). Because otherwise it's not of much use anymore due to knowing the games already and therefore having much less risk of getting in situations where you'd even need this.

I think if it works in reverse as well, as in you do a little extra damage if an enemy would have a sliver of health left after a hit. I've noticed that when they fight against each other they can be left with a sliver of health (and still hit max strength of course). Yet when I do my sneak strike attacks, there doesn't seem to be anything between 10-20% left and nothing. With all the time spend I've never had a hit where an enemy was left with just a pixel of health left. (Like what a Stalnox remains at if you don't hit its eye) A gimme, close enough, enemy dead. Very nice!

The last 20-25% of the Depths I have remaining is a big maze. I guess I'm under the jungle/tropical biome, lot of water there, thus lots of walls and a lot of backtracking. Now I wonder whether Zonaite regenerates as well, seems to be as I keep finding more empty camps with Zonaite repopulated. The boxes are still empty of loot and the enemies still gone, just Zonaite statues. I haven't seen anything come back in the mines yet, but I tend to go around them when backtracking as they are always in a deep hole.

At least kills of static enemies have been permanent so far!