That's reasonably fair. A lot of things could mess up the classic game, some even earlier than that, but certainly by the time you were using Enchant an Item and it's ilk, there'd be issues. In the classic game. And, not surprisingly since it evokes the classic game so well, in 5e, of course.
I'd have to review the spell list for 1e, but the real game breakers tend to be things like Wall of Stone, Harm, Find the Path, Disintegrate, Word of Recall, Feeblemind, Death Spell, Anti-Magic Shell, Hold Monster, Teleport, etc. Add on top of that the problem that damage from spells like magic missile or fireball don't cap, and by 13th level they are starting to become real problems. Very few creatures actually have much more than 13d6 hit points.
For 3.0e, the list of problem spells probably isn't that different, because 3.0 ported the spells from 1e rather faithfully. The breaks tend to be things like Haste, which no longer aged you 2 years every time you cast it, and the fixes tended to be things like Fireball, which was now capped in damage. The list of broken spells gets much longer starting in 3.5 as the 3.5 spell revisions in general broke spells wide open by a ton of unplay tested shenanigans and changes for formalist rather than play reasons (such as Ray of Weakness no longer allowing a saving throw), adding in particular the shapechanging spells like Alter Self and Polymorph to the list of things which were completely broken straight out of the box.
In 3.x, by the time you had 4th level spells you were quite possibly seeing real problems. Thus E6.
I think E6 was about far more than just ensuring balance between the spell-casters and non-casters. For example, a big reason for stopping at E6 was to avoid issues like 'Raise Dead' being readily available, but perhaps an even bigger reason is simply just to keep the game feeling 'gritty' with less gap between a 1HD orc (for example) and the most potent PC's in the game. At least in the core, there is not a lot that is outright broken and game changing at 4th level spells compared to 2nd and 3rd level spells, and you could easily take it to E8 if your main concern was class balance. Polymorph is the only thing that comes to mind, and it was broken open only at 3.5e. The really big game changers other than invisibility and fly are at 5th and 6th.
But, right or wrong, you just can't say "for all editions" and "6th level spells" in the same breath
Ok, you are right. I know virtually nothing about 4e, and as far as I can tell it was never broken in the same manner that prior editions had been.
True in the classic game, just getting out of the lowest levels is quite a hurdle. But in the WotC era survival into high levels is not that challenging (for the players) unless the DM decides to go well beyond the guidelines...
Unless you are heavily character optimizing, 3.X in many ways is more dangerous at high levels than prior editions. One major reason for this is that in 1e your savings throws started to become reliable right about the time that what you were saving against was more and more 'death'. You could count over time on your saves getting better. But in 3.X over time all but your best save(s) got worse relative to the DC of the things you were saving against, which mean that just about the time you were encountering save or die, your saving throws now highly unreliable. The only solution to this was to kit up with a suite of invulnerabilities like death ward, heroes feast, and freedom of action and loading up with a standard Christmas tree of items that boosted saving throws. Not 'going beyond the guidelines' also involved not abusing this hole in the 3.X math and also allowing the Christmas tree by allowing fungible magic items (or placing the trees in the path of the players).