Interesting question.
In Classic D&D (I'm thinking mostly 1e, since I played it so heavily, but tapered off in the 2e years, and only briefly played Basic), most monsters didn't have a lot of hps relative to what PCs could dish out, so a 'solo' fight against a straightforward monster was usually short. The thing is, the game was full of monsters with immunities and tricks and gotchyas, so it was rarely straightforward. A non-trivial combat vs a lone monster would consist of many rounds 'wasted' on attacks, spells, and actions that were misdirected by some trick, missed, stopped by magic resistance, or otherwise accomplished little or nothing. Eventually you'd figure out how to kill the thing - or it'd kill you.
3e, a lone monster was the baseline same-CR-as-level encounter. They still tended to be over quick, but the monsters could do so much damage or have such devastating special attacks that it could take a PC or two down in the one or two rounds it lasted. Not quite the 'rocket tag' of high-level casters, but suggestive of it.
4e, had some monsters that were specifically designed to be solos. Initially, they were just big bags of hps that took a while to empty and were, in spite of a +5 save bonus, sadly susceptible to being locked down. Later they were given more and more interesting offense and action-preservation tricks that made Solo encounters as challenging and interesting as set-piece ones.
5e we've already heard about, of course. In theory you can use a higher CR monster as a solo, in practice Bounded Accuracy either lets you burn it down fast or it's high damage or some other nastiness overwhelms you.
OTOH, Legendary monsters are, like 4e solos, designed specifically to challenge a whole party and have powers to give them more & more interesting offense and action preservation including Legendary Actions.
But, they're still up against the advantage Bounded Accuracy gives to the numerically superior side of any battle.
