To take an example from the video above, let's say a player wants their character to take just a sip of a potion. How should a game handle this to maximize player agency? How should the GM handle this to maximize player agency?
This is a very particular example, in that it rests on a premise that it is a meaningful part of play that the players will have their characters (i) discovering, and (ii) hoping to take as their own, unidentified but most likely valuable magical consumables.
Of course there is a reason that we can all recognise and not question that premise - it is implicit in a very traditional mode of D&D play, of exploring dungeons (or similar sorts of places) with the goal of taking their contents; and we know that magical potions are one important element in those contents.
But I think it is worth making the premise explicit, because it is the premise that will help us address the questions you ask: in a game of map-and-key exploration, where the players are hoping to acquire information - including information about potentially valuable magic - that is initially secret from them (ie it is in the GM's notes) but that they can oblige the GM to reveal via appropriate action declarations, what counts as
appropriate? Is saying "I sip from the unmarked bottle (which I suspect is a valuable magic item)" an appropriate action declaration - you can learn about a possible treasure, at the risk of it being a poison - or a foolish one - you don't drink enough to get any magical benefit, but you sufficiently reduce the dosage such that no one else can benefit from it either?
I don't see how there can possibly be an a priori answer to this question. It depends so heavily on conventions of play. It's like asking, Is it fair for the GM to have a chest of gold coins where the coins are in fact silver coins very lightly electroplated in gold? Or, Are Mimics fair monsters? Or, Is it fair to have a secret door which can only be discovered by tapping the wall to find the hollow bit, but the wall is covered in invisible yellow mould such that as soon as you tap it you trigger the mould's deadly spores? And suppose a player decides to have their PC sniff the air near the dungeon walls to try and identify the presence of deadly moulds, is that a clever tactic to be rewarded, or a foolish tactic to be punished? -
save vs the spores with a penalty to reflect the fact you're deliberately and deeply inhaling the toxic air!
On my quick review of the original D&D books I didn't see any discussion of sipping potions. But it is clear that, at some point or other, a convention emerged at Gygax's table, because his DMG says (p 125) "even a small taste should suffice to identify a potion in some way - even if just a slight urge." But then the very next sentence is "you should add a few different sorts of potions, both helpful and harmful, of such nature as to cause difficulties in identification." In other words, having decided that sipping potions counts as sensible rather than foolish play, he is immediately suggesting the GM add new elements to the fiction to reduce if not negate the informational benefits of the tactic. This is the "arms race" dynamic of one person's table, and its conventions, being presented as advice to the world at large!
Moldvay Basic (p B47) doesn't manifest the arms race, and notes the tactic only in passing: "A character can only identify the exact type of [magic] item by testing it (trying on the ring, sipping the potion, etc)."
Torchbearer is a modern game inspired by these classic games, and it has the following clear statement (Scholar's Guide, p 162):
If you sip a potion, you can discern the potion’s effect. Sipping does not diminish the number of draughts or effects of the potion. Note that if a poison potion is sipped, the character takes the full effect of the poison (as described).
The Torchbearer designers here are not simply stating a convention of their play as if it were uniformly applicable. They're stating a rule with the deliberate goal of creating a play experience that emulates the Gygaxian experience at a certain point in the evolution of the Gygaxian conventions.