Interesting. If we were to adjust the dials on encounter creation so that every fight (especially boss fights) assumes the PCs are at 100% power, what should we do differently?
Is there anything you can do differently?
Consider that there is no "reduced power" statblock for any creature. Or that the CR system doesn't take anything like reduced party resources into account. If an encounter is ECL 7, it stays ECL 7 whether it's the first or last fight of the day. There's no way the DM can know, in advance, what state the party will be in when they reach the final encounter of an adventuring day. They could be down a man and struggling to survive. They could be at 50% power, 75% power, or hey, even 100% if they found a way to sneak a long rest in somehow.
The only way a DM can adjust for this is with quantum ogres, fudging die rolls, or other "behind the scenes" manipulation- if you're a "living world" DM, ie, you believe the campaign world is set in stone and shouldn't change for players, then inevitably the party will just fail due to bad luck (conversely, they might trivialize the adventure's challenge due to good luck, but whether or not this is as problematic depends on your point of view).
There is no guideline for the DM for how to do this, really, it's pretty much play it by ear. Conversely, it's the same for the players. There is no advice saying "don't use Action Surge in this fight" or "try to only cast one spell per encounter". You use the resources you have to in order to win. If this means that an encounter implodes because nobody can save against Hypnotic Pattern or that you waste a Fireball on resistant enemies, you'll have more or less resources for the upcoming encounters.
It's perfectly believable that a party could suffer misfortune so significant that they cannot proceed and are stuck on encounter 4 out of 7. Of course they want to rest, they have to! But if "long rest" is a naughty word in the adventure's design, then what?
The problem with resting really is that the DM wants to control when it happens, and it's not really a decision they should get to make- they can discourage it, but if the party believes they cannot face another tough encounter, they're going to attempt to rest (or retreat).
And it's not like monsters are designed to have reduced resources when you encounter them either. Ask yourself when the last time you saw a monster in a fight have reduced hit points or a spellcaster is down a few spell slot. I'm guessing that chances are, this is a rare occurrence, since doing this should lower the difficulty of the encounter, and it's not clear by how much. The CR system isn't built that way!
Let's say you decide a second level party is going to fight three Wights in the broad daylight, so that all their attacks are rolled at disadvantage. Does this make the encounter appropriate? Some would say yes, some would say no, the truth is, nobody knows, because it all depends on the roll of the dice. You can attempt to use statistics, but statistical anomalies happen. Anything you do is an educated guess.
Encounter design has always been borked, as the closest anyone has ever come to answering this was loose advice like "adjust the xp award if a fight is too easy or too hard". The impact on player resources, the ability for them to actually face 6-8 encounters between long rests, none of this is taken into account.
What's also missing is any real incentive for players to push their luck. If you don't have to face a dragon at 75% power, why would you? Why would anyone, that just sounds suicidal! And yet the game is designed with the idea that of course people will do that, and what guidance is there for the DM about how to handle that issue?
There's a culture of disdain for players who try to rest whenever they can, but the fact is, if the party goes into a fight at 50% power and the Cleric takes a breath weapon to the face and is dropped to 0 hit points in the first turn, how many people would say "the players were dumb to not rest first"?