We've done this song and dance before about whether or not Nintendo is truly supporting the 3DS after the Switch took off, but Nintendo keeps saying that they're committed to it. A few times a year an executive or financial briefing will pop up and tell people that they're still in it for the long haul, which is what's happening this week with Nintendo's updated financial briefing for the fiscal year ending in March 2019.
Nintendo explains that the 3DS is "set apart" from the Switch by being "affordable, portable and lightweight." They cement that the "price-friendly" nature of the system is why it's still a part of the "grand scheme" of the company and explain that more "evergreen titles" will arrive in the future, including more in the "turn of the year." If Nintendo has an end date in mind for the 3DS it's not right now and they're not sharing it. The real test will be E3 2019, which will give us a glimpse into what they want out of the portable in 2020 (if anything).
Remember: Nintendo has also said that if the Switch continues to sell "multiple units per household," that they'll shift gears. Also, a price drop sometime in 2019 is not a wild assumption, which would lessen the gap between the more expensive Switch and the "affordable" 3DS.
My take as a lot of this is too clandestine to make a definitive statement on: the 3DS rocks and if it died tomorrow it lived a good life.
Financial Results Briefing [Nintendo]