billd91
Not your screen monkey (he/him) 🇺🇦🇵🇸🏳️⚧️
I think assumptions about play changed - from the idea that a group (led by the GM) would make of this stuff what they wanted to, based on their familiarity with existing fantasy, myth and legend, to the idea that part of what was fun about the game was playing in an already-authored canonical setting. You can see the beginning of that change in AD&D, but it really emerges in the mid-80s (look at the different nature of the flavour text in Unearthed Arcana and Oriental Adventures, for instance) and then grows over time.
Assumptions certainly did change, but not just about play, about the players. At some point, the publishers of D&D could no longer assume that the players were as steeped in the fantasy literature cited in the 1e DMG appendix as the designers and I think they realized that. They could no longer just assume everyone had read the same books. Some players were coming to D&D first rather than the literature first. D&D had to provide its own context more and more rather than refer to one out there due to fantasy cultural literacy.