If by solved you mean it's a feature you're required to support... It can never be truly seamless when things like wall clock or device state (SD card is missing suddenly) or network connections disappear.

Nintendo, like all other platform-owners (e.g. Meta/Quest, Sony, Microsoft) is VERY strict about games released on their platform and have very strict requirements before anything is allowed to be sold with the Nintendo label. I very highly doubt they let devs NOT implement the pause ability. AFAIK you can't just OS-pause a game and expect it to run fine when it resumes, there are soooo many systems at play: animation, physics, sound, input, etc. that need to be cleanly stopped/resumed that I doubt it's as easy as just OS-pausing.

That is different, because you can't interact with the game anymore. In game pause can let you change your settings or map for example. Menus and map are still running in the game loop, so then you need to make they get the input events but not the gameplay part.