Nintendo has announced that starting later this year, you'll no longer be able to use overseas-issued payment cards or PayPal accounts in order to pay for content from the Japanese Switch eShop.
In a notice on the Japanese Nintendo website (thanks, VGC), the gaming giant says that in order to "prevent fraudulent use" (we're using machine translation for the post, which is originally in Japanese", overseas-issued cards and PayPal accounts will be banned.
It looks as though PayPal accounts opened in Japan, as well as Japan-issued credit cards, will still be valid. Unfortunately, though, if you've been using the Japanese eShop from outside Japan, it looks like you might have to find a new payment method.

It's common for Switch players to buy their games from the Japanese eShop, either to circumvent what they see as poor exchange rates or to get games that they can't get in their native countries.
Nintendo's reference to "fraudulent" behavior might be a veiled nod to this practice; it might seem relatively harmless to me, but Nintendo is well-known for what could charitably called "fiercely protecting its products", so I wouldn't be surprised if that was the reasoning.
So, what does this mean for you if you're a frequent user of the Japanese eShop? Well, it means you'll need to find some way of using a Japan-issued payment method if you want to continue, unfortunately.
This decision shouldn't affect your country's native eShop, and it also likely won't change anything if you're already using a Japanese payment method to access the eShop.

Earlier this month, Nintendo officially announced the Switch's successor, putting an end to months of speculation and leaks regarding the device's appearance.
We still don't know what kind of specs the machine will have, though; we'll have to wait until April's forthcoming Nintendo Direct in order to learn that.
Still, with a new device coming, perhaps it's no wonder Nintendo is tightening its rules a little. Stay tuned for more. In the meantime, the Japanese eShop payment crackdown takes effect on March 25th.