The multiplayer for Stardew Valley on the Nintendo Switch is in QA (Quality Assurance) at Nintendo.
QA?
This means that the development is finished and that Nintendo is currently looking at possible errors or problems in the upcoming multiplayer update.
If Nintendo does not see any strange things, then it is very likely that the update will be coming our way soon. Developer ConcernedApe reports in any case that they will report it as soon as possible if more information is available.
The Stardew Valley multiplayer update for Nintendo Switch is now in QA. Will let you all know ASAP when more concrete info becomes available!
— ConcernedApe (@ConcernedApe) October 2, 2018
Are you already looking forward to the multiplayer update for Stardew Valley?
Stardew Valley has been available for the Nintendo Switch since October 5, 2017.