UPDATE 23/3/22: Nintendo has finally commented on the Wii and DSi eShop channels, after days of unexplained downtime and increasing fan concern over their future.
In a statement to Eurogamer this morning, Nintendo said the storefronts were "undergoing maintenance", with no word on when they might return.
"The Wii Shop Channel and Nintendo DSi Shop are currently undergoing maintenance," a Nintendo spokesperson said. "We will provide an update at a later date."
Prior to the storefronts' disappearance, Nintendo seemingly made no mention of maintenance plans.
ORIGINAL STORY 20/3/22: It looks as though both the Nintendo Wii Shop and the Nintendo DSi Shop channels have been taken offline.
With no prior notice from Nintendo, players aren't sure if this is unscheduled maintenance or a sign the stores may have been removed for good. At the time of writing, it's thought the stores have been down since at least 16th March.
While players haven't been able to buy new content from either store for awhile now, the unavailability of them means players who've already bought digital goods are currently unable to retrieve them.
Perhaps most interestingly of all, whilst the stores show error codes - 290502 and 209601 for each respectively - some sources report that Nintendo's main server is still up, so games can still be downloaded via third-party tools like NUSDownloader, intimating that it's just the frontend sites themselves that have been taken offline (thanks, gbatemp via ResetEra).
As yet, Nintendo has not publicly commented on the outage.
Nintendo's Russian eShop was "temporarily placed into maintenance mode" recently after its payment service provider suspended the processing of payments in rubles.
A Google translated version of the statement on the eShop still reads: "Due to the fact that the payment service used in Nintendo eShop has suspended the processing of payments in rubles, Nintendo eShop in Russia is temporarily placed into maintenance mode".
It's worth noting this temporary suspension of service doesn't appear to be a result of Nintendo's own actions, but rather that of a third-party. Unlike Microsoft, EA, and CD Projekt - all of which have ceased sales of their products in Russia while the country's invasion of Ukraine continues - Nintendo has yet to publicly comment on the matter.