There's already a rule related to game-related communications (i.e., PM's from the GMs). I suppose "timestamp" could easily be classified as "text content" and thus already covered:
Otherwise you just re-write that rule to say something like:worcej wrote: ↑Fri Oct 23, 2020 3:30 pmGENERAL RULES
0. The GMs reserve the right to adjudicate individual scenarios that arise during the course of the game at their discretion.
1. All players must vote at least once per game day and make two posts per day phase. There are no posting requirements for the night phase.
2. All players are forbidden from posting screenshots or text contents of ANY game-related communications.
2. All players are forbidden from posting screenshots, contents, metadata, of ANY game-related communications.