There are several questions on BCG Stack Exchange that ask about information which was previously common but "vanishes", even though it can be deduced from other common information.
Correct opponent's assumption about my morph? has an answer that yes, a player must clarify the identity of a facedown card where that identity was previously public.
Shuffle facedown cards to hide revealed card? has an emphatically negative answer, the opponent can't be forced to play three-card-monty.
Prove a creature has morph? - the answers indicate that yes, eventually, it must be, and you can't deliberately obfuscate this fact after the game.
The answers generally indicate that the opponent must be able to deduce the card identity from what they knew before combined with what they know now ("derived information") and one of the questions involves a situation with coloured markers that happen to coincide with the card's colour identity.
Is there anything that disallows or allows or even requires markers to be used to indicate which of a group of facedown cards previously had a particular identity? Even further, is the player allowed to place markers on cards under the opponent's control to assist in tracking these previously known identities? An answer for any rules enforcement level is acceptable. This is most likely to be an argument at Regular, but would also have importance at Competitive and Professional.
For example, another question asked about whether the fact that a particular card was the commander remained public even if the card is facedown. Such a card could be marked with a C token to remind players of that fact. Other situations involve revealing a card that the opponent would want to ensure they can track in case they obtain a means of removing it later, and want to place a target-shaped marker on it.