The deadliest monster shall always be the scheduling conflict.

The Gas Spore Fungus sounds like a great start to a mini adventure. Oh no, you've all been hit by it's poison and will die in [1d12] hours. The local healer can't help you, but she hears of a cave spring that can give the effects of Lesser Restoration. Sadly, the cave system is infested with other monsters and hazards. Best of luck, adventurers

Shadow Dragon would go hard attacking a populated place. Like it gets a bunch of commoners in that breath, congrats you have an army of shadows to deal with now.

I like the idea of a Solar choosing to fight up close to keep things fair or because they want to show mercy, but then the moment they decide someone needs to die they fly straight up, and that's everyone's cue to flee or surrender.

I won't die.  I'm a goldfish.

I once hit a character with a Gas Spore Fungus while they were clearing out an infested mine, and when they returned to the tavern to get their reward, they got caught in a Groundhog Day style time loop 'whodunnit' mystery situation. The loop would reset every hour, and since the players had come from outside the loop, their time was not looping.  I counted loops as they progressed through the mystery, getting more and more desperate to solve it before their companion hit their time limit.  Very fun!

Don't forget Mummy Lords spoil all food and drink in their lair cannonically, so your party won't have any food or water to resist exhaustion after...

Who remembers the old days when vampires would drain character levels when they hit

to be honest, as a player and as a GM, i allways loved the deadlyness of the oldschool monsters. The fear you could bring into the Players if they realize that they are about to face a ghost was great. Also, i it was very rare that players made a fuzz if there char died.

The long rest thing is 100% intended to cause exhaustion

Shadows are always scary. Death House, anyone?

7:11 a beholder, a mindflayer archanist and a gibbering mouther walk into a bar

There was a giant mosquito that drained a constitution in 3.0, and the vampire did as well. Normal Shadows used to all so in 3.0 kill you and turn you in the shadows.

So, my advice to players would be to take advantage of the new explicit rules for making spell scrolls. The Acolyte back ground gives you proficiency with calligraphers tools. So, a Cleric with the Acolyte background can innately scribe scrolls of Greater/Lessor Restoration and Remove Curse. Assuming you have the time and resources to prepare these before you need them, you can have those spells available for your party without having to take space in your prepared spell list.

A surprisingly deadly monster I came across is the lowly Bugbear. When running Defiance in Phlan I had a group sneak into the goblin lair and wantonly kill the goblins. The Bugbear leader got wise and hid from the party to try to surprise them. The Rogue went to free the hostages and failed to perceive the leader. He met an untimely demise with the Bugbear’s Morningstar with Surprise Attack dealing enough damage to kill him outright in a single attack. Would only happen to like a level 1 party but still watch out for enemies with tactics and abilities to go with them. They’ll get you.

If my Bear Totem Barbarian didn't have his Potion of Giant Strength, I definitely would've died from a Greater Shadow encounter, with his 4 shadow minions. Oh, and it was a double encounter because a paladin was a bit hasty and decided to solve a riddle by ignoring it and pushing the sarcophagus wide open. Nightmare! xD

In one of the campaigns I run, the party had a side adventure where they faced yuan-ti for the first time with the big fight at the end containing a yuan-ti anathema. The gasps when the wizard went down and then got curb stomped by the yuan-ti causing 4 failed death saves was priceless. It was the first time that they had run into anything that "finished" the kill before moving on and really took them by surprise. They were victorious and the cleric had revivify prepared so the wizard was returned to life, but they still talk about that fight and are get really nervous anytime I mention yuan-ti in game.

You also forgot the encephalon cluster which if it reduces you to 0 you simply die.

7:55 that happened to me in 5e with the wraith. The rest of the party locked in after my cleric died and couldn’t be healed, and then was raised as a specter next turn. It really is a game changer when the monsters say “no revive for you”.

The rakshasa curse is gonna be deadly if your divine healer doesn't have remove curse at the ready.