I've been playing Grim Dawn lately, in addition to a little Deep Rock Galactic. I'm getting up towards "endgame" in both of them. I had never really considered "endgame" in the context of RPG-like games with leveling / progression systems (as opposed to, say, grand strategy games where "endgame" is when your strategy has come to fruition, your position is secure, and you get to coast to victory). The way it seems to be used in Grim Dawn and DRG is that your progression stops or slows, but difficult and very time-consuming content remains, to interact with optionally.
Looking at Basic D&D through this lens of CRPG terminology, it seems like name level (9th-10th) is almost a soft-cap. The XP to level changes from exponential to linear (with a steep slope), the rate of HP gain is halved, you've gotten most of the attack throw and save improvement that you're going to get, and you start getting access to new, expensive content on long time-scales: domains.
There are problems with this model, mostly around MUs getting access to 6th level spells at 11th, and some thief skills still don't get up into the 90+% range until 12th - but switching from 5% improvements to 1% or 2% improvements is a very soft-cap "diminishing returns" change of progression structure. I don't think I would mind a variation that made this more explicit, by making 6th level MU spells ritual magic, and compressing thief skill advancement so that eg Hide in Shadows did get up around 85% by 9th level and then improve by about 2% per level thereafter.
Incidentally, having very-fine-grained progress on thief skills post-9th level might be the best argument I've ever considered of for using percentile thief skills rather than d20 or d6.
I think it would be reasonable to think of making it into the 9th-11th level range as "winning" at D&D. You've made it over the hump and fulfilled the default goal of accumulating personal power; further efforts to accumulate personal power will be slow going. But now you have enough power to pick your own goals. Or you could just retire to your tower and start a new character.
I've noticed among Grim Dawn players a sort of division, between players who enjoy leveling characters, and players who rush through leveling to focus on endgame stuff. I think that (say) my past ACKS players also divided naturally into these two categories. For some of them, domains were the game and leveling was just something you did to get there. Other felts compelled to get domains just to keep up with the endgamers in terms of domain XP, but had no interest in domains as ends to themselves.
I think in a "MEANINGFUL CAMPAIGN" with very high individual-player autonomy, having a few players pursue the domain endgame is probably less disruptive for everyone else than in a high-cohesion tight-party game. Particularly without rules for XP from domains. And if domains are explicitly endgame content, and at that point you're already about capped-out on XP progression anyway, who cares if they give XP or not? The passage of time that Gygax describes, where you're probably only passing a couple weeks of game-time per week of real-time, also seems likely to keep adventuring PCs at the center of the action, while PCs hoping to only do domain stuff will be stuck waiting a lot.
... I wonder what the 1e DMG has to say about domains specifically?