Here’s the rub. The argument seems to be that the defaults that are already in the DMG aren’t prescriptive, but any new information added must be prescriptive.
So, for instance, 20 pages in a 300 page book describing the default D&D cosmology (including a full half-page illustration of the cosmology) isn’t prescriptive, because some alternatives get a one paragraph write-up.
On the other hand, language like:
“Some people prefer a more OSR-style game. If you want to try this, here are some rule variants that taken together restrict healing (and can increase the lethality of the campaign), and put a greater emphasis on exploration (including removing certain spells)” is considered prescriptive, because whatever is indicated will become the new standard.
At the end of the day, it is “appeal ad conservatum”. Change is viewed with suspicion, because it’s change.
There are only so many options the book can include. Why would they actively encourage a previous edition? How much text would you have to include to have a section on how to make the current edition feel more like an older one? I think it would be confusing to add such a discussion without significant game theory discussions that are beyond the scope of a book written for people who want to play the current edition.
Unless you are already familiar with OSR style concepts, it's not at all obvious what it means much less why you would want to do such a thing. While there's an active OSR community, it's a tiny fraction of the entire player base. Fortunately if you want an OSR version of D&D there are plenty of options out there, I just don't see why a discussion of it belongs in a book dedicated to the current edition.
There is a discussion in the DMG in the Dungeon Master's Workshop chapter that covers several options to make your game feel more old school from alternative rest rules to limited healing to lingering injuries. If you want to go beyond that there are other resource.