And, after the 3DS got its recent system update, this time it looks like it's the Switch's turn. Firmware 9.1.0 has been released by Nintendo for all regions and the changelog states it's meant to fix an issue with the colour animation not showing correctly when attaching a Joy-Con to the console... alongside with the usual stability. However, there may also be something else under the hood, as @SciresM has warned that TrustZone might have received an undocumented change, so current CFWs may break if you install the update.
Ver. 9.1.0 (Released December 4, 2019)
General system stability improvements to enhance the user's experience, including a solution for the following:
Resolved an issue where the color animation was not displaying correctly when attaching a Joy-Con controller to the Nintendo Switch console.
Needless to say, given those premises, it may be best to avoid updating for the moment if you're running a CFW or if you wish to mod your console. Not much else is known at the time of writing - this post will be updated with any useful hacking and homebrew-related info.