^^
More than that, the metrics run on both sides of the screen.
The 'correct' solution is a synthesis of frequency, optimization and challenge varieties.
GM side variables, based on the campaign. If you're a player, this determines what kind of preparedness you need. If you're the GM, this determines what degree do your players need to be protected to function in your game. Preparedness more than 1 step above or below the campaign can be contrary to fun, since you wind up just ignoring pretty much everything incoming, invest more than you would have lost to begin with, or wind up being Kenny from South Park.
Usage of 'true' Save or Die - The metric here is whether they show up at all.
-None: Business as usual.
-Low frequency(1 in 12 encounters): Pack post-ante recovery like Break Enchantment, Stone to Flesh, Revenance+Revivify, etc. Your regular defenses can deal with them for the most part. It costs a bit of xp and wealth, but fits easily into disposable wealth. Presuming 1-2 encounters per session, this represents a sizable chunk of a campaign arc(and if you are any good at offense/defense, you probably won't lose anyone). If you're going by entirely random monster picks, you'd probably end up here.
-Medium frequency(1 in 8 encounters): Frequent enough that generic rerolls or short duration immunities should be used. You don't need to walk around all day warded against death, but its cost effective to have a few scrolls or potions to reroll bad saves or put up Death Ward for a fight rather than just getting splatted on a regular basis. Contingencies and contingent style effects like Heart of Water may be useful at this point to negate particularly nasty examples.
-High frequency(1 in 4 encounters): Once a day IC, something may flat out kill you. You're going to need immunities, long term ones, and contingencies for things that bypass those(e.g. Implosion). Immediate action options like Wings of Cover or the Celerity series to outright avoid having to save to begin with are vital as well, along with surprise negating senses like Mindsight, and high initiatives.
-Nintendo Hard(every encounter): Every fight has something of this variety(did you attack a mage academy or something?). Using extreme no-sell tactics like tinfoil hats, shaped antimagic fields, Astral Projected adventuring and the like to provide persistent all-inclusive immunities would be needed to survive at all.
These effects include death, disintegration, permanent transformation, permanent mental alteration, permanent immobility, etc. As long as an effect is permanent until removed, and invalidates a character's ability to participate, it counts here. So severe debuffs like Bestow Curse/Blindness don't count, as you can participate, if in a crippled manner, but being polymorphed into a bunny would.
Usage of Save or Lose - The metric here is how many characters they might eliminate. Turncoat effects count for twice as much. Worst enemy of a PC is another PC.
-None: No worries
-Low(1/4 characters): Most even marginally optimized parties can kick the enemy's ass just fine with one man down. If the party is sufficiently optimized to be able to take out half(or more) of an equal CR enemy in one round, they can just let the fallen dude stay there and finish the enemy off anyway.
-Medium(2/4 characters): Some trouble here, as it implies AoE effects with a good DC(taking out the weak saves in one shot) or mind control. Straight offense is still an option, but you'd need to reliably oneshot the enemy. Recovery is significant here, undoing the removal or breaking mind control needs to be available, and more importantly, available to multiple characters(potions) or available without actions(contingents).
-High(3/4 characters): Unless you have wholly embraced rocket tag and is able to destroy everything solo in a single round, you're looking at a TPK here. Mass recovery is helpful, but generally not all that available unless you can do it with Dispel Magic or Magic Circle. So immunities on at least half the party is a good idea. About the same as a SoD of one step lower(see above).
-All(Everyone): This is just an Save or TPK, and should be treated like an SoD of one step higher(see above). Post ante methods do not work on this. Also note that frequency here relates to odds of success, unless the DC is improbably high or theres no save at all, it is extremely improbable to face one that can successfully wipe a whole party.
These are the non-permanent defeat effects like Stun, Sleep, involuntary teleportation, mind control, etc. They might be worse for a given encounter, but
Nonstandard Loss Track - The criteria here is the number of hits you can take before you are out.
None: Nothing to see here.
Low(Requires upwards of 3 rounds) - As with SoL, you can just damage race them easily.
Medium(2 rounds) - Damage racing is still an option, but effective recovery(canceling out as much variant damage as at least 1 round of the enemy can inflict) is good as most of these are also debuffs.
High(1 round) - Save or Die/Lose by another name. See above.
These are the alternate non-hp tracks you can go down from. Ability damage, Negative levels, etc. The danger is mainly from having crippling overspecialization leave you with a vulnerability, such as the famous Dragon vs Shivering Touch example. Ego whip is another nasty sample.
Statistics Race - This is just damage vs hp and DR, to hit vs AC. Standard optimization applies and I doubt I need to elaborate.
Armed with this information, you can then have a DM pick the desired hazard level of each, and ensure the party is ready to the appropriate degrees.
EDIT: Readable
