This neatly serves to point out that there's different types of balance:
--- balance between characters in here-and-now play situations (as in, how equal can everyone's contribution be in this combat or that social encounter or some other exploration piece)
--- balance between characters on a here-and-now mechanical level (as in, if you had the charactes all throw down against each other, would each one have a roughly equal chance of winning or would one of them win every time)
--- ongoing balance between the characters/party and the game world (is the party stomping everything it meets, or are they constantly getting stomped, or...)
--- medium-term balance between characters (i.e. are there certain types of adventures or opponents that put some classes at a big advantage or disadvantage in dealing with them and does the campaign give each character a chance to shine)
--- long-term balance between characters (as in the 1e Magic User, where you suck now in order to potentially dominate later, or the 1e Ranger where you dominate early but (IME anyway) really run out of steam as the levels go by)
When thinking about RPG balance I generally only think of the first two and consider them as generally the same thing.
In combat does everyone have the opportunity to meaningfully participate roughly equally in their specific individual way whether that is tanking, debuffing, buffing, controlling, striking, whatever.
That was the design goal in 3e, though often not achieved, it was fairly well achieved in 4e, and though not as tightly achieved in 5e as in 4e it was still decently achieved in 5e.
I don't care about who wins a fight between PCs, I care about them feeling they are all contributing parts of the team and not a bench player second string part of the team.
Balanced encounters for their level is not usually what I think people mean when they say RPG balance generally.
The medium term, consideration of having a variety of adventures where it can rotate who shines and who are knee-capped (anti-magic adventure, all urban, all wilderness, no equipment prisoner break out, all social, heavy undead, etc.) I don't really think of as part of the balance question except tangentially. Sidelining wizards and sorcerers entirely for an adventure with anti-magic is not something I would generally think of as a balancing factor that justifies them being baseline overpowered, or that monks do better in a no equipment jail break does not mean their baseline should be handicapped.
The long term one I think is generally a poor idea of balance because it does not apply so many times. One shot low level games. One shot high level games. Campaigns where characters die so low level weak characters never get off the ground, or beginning strong ones get traded in for ones who are stronger later. Also 1e halflings and thieves get the short end of the stick in general.
Similarly the 1e "balance" of gatekeeping strong character classes behind requirements for those who have randomly generated high stats which already make characters stronger.