Alternatively, maybe the user is talking about a European release date or a regional code. For example, some updates were regional and had different version numbers. But that's stretching it.

Another possibility is that the user is referring to a game update that happened on 2021, January 21st, hence 21.1, or January 21st being the first update of the year. But that's speculative.

Alternatively, maybe it's part of the game's internal numbering. For example, some games use version numbers like 1.0, 1.1, up to 2.0, etc. If the game is past version 2.x, then 21.1 might be a misinterpretation. However, given the typical versioning, such a high number is unlikely. Instead, it's more plausible that the user made a mistake in the version number. For example, "21.1" might refer to a patch that was released in the first phase of 2021 (hence 21.1) or as a typo for 1.21 or 2.1 or 3.1.