So far there's nothing that indicates a continuous support to evergreen titles; surprisingly, it's even smaller titles that receive updates like Capitan Todd. Mario Party severely lacks content yet nothing has been announced so far in that regard (that one really surprises me). Splatoon 2's content updates have stopped for 3 months and there is nothing indicating that they would release more stuff for that game (and if they did, that would be really WEIRD after announcing that December would be the final content update). Mario Kart has been out for nearly 2 years without anything new other than Labo compatibility, and it increasingly feels like the ship has sailed for new content (we were already saying that 6 months ago). Mario Odyssey receiving any update is also very very unlikely at this point; the game is also less green everyday and the combo NSMBU + Mario Maker 2 will probably reinforce this trend. We are already hearing stuff about the next Zelda for 2021...
Continuous support of those titles may eventually happen, we never know with Nintendo. But right now, it is far far far (far) more likely that Nintendo is in the middle of developing sequels for those games, or new unrelated games.
My bet is, since last year, that Mario, Mario Kart, Zelda and Splatoon would all have completely new games during the 2020-21 period (I can see Zelda being 2020 and Mario being 2021 for example, with 3D world releasing in 2020 for the yearly Mario fix). I can see Mario Kart 9 being introduced in 2020; maybe with an adventure mode, which in my opinion is way overdue and a good way to justify a new episode that wouldn't feel incremental. Splatoon 3, which I also predicted for 2020, is a little trickier. They could of course add new stages and weapons, maybe 1 a new mode, but I don't think they would get away with a 2.5 release and SPlatoon 2 had TONS of new stuff as compared to the first in a hindsight. I personally would prefer to receive expansions, but as the Octo-expansion showed us, those do not sell new base games.