What is the one rule tweak that one could make to 'new school D&D' (let's say, 3rd, 4th and 5th editions) to encourage old school play? Let's say one's aim was to do the least violence to the system by making the fewest changes imaginable - keeping everything the same but altering just one single rule. What would that be?
What trips off the tongue immediately is XP for gold. But I want to make the case that the one change in question would be going back to actually rolling the dice for starting hit points, as opposed to beginning with the maximum hp available.
Starting with maximum hp as of right, as most editions of D&D have recommended or mandated since 2nd edition, is in my view possibly the most corrupting single rule from an 'old school' perspective, because of the way it transforms expectations. Maximum starting hit points makes the PCs more resilient, of course - they can survive more. But it also enshrines the expectation that they are tougher or more special than NPCs or monsters of the same broad rank. It bakes in 'plot armour': this is a story and these are the main characters, rather than the Just Another Adventurer archetype that old school games assume.
And it bolsters the sense that there is something wrong or perverse about character death - like it is something to be avoided at all costs rather than the natural consequence of taking risks in the game (or mere foolishness). The idea that players should be protected from having hurt feelings because their new character has died is, frankly, patronising; far better the excitement that comes from knowing that dice rolls matter from the start.