An 11-speed drivetrain is still perfectly capable for most mountain biking. You get 11 rear cogs with a wide enough gear range to handle steep climbs and fast descents, and many riders won't notice the difference from 12-speed in everyday trail riding.
Where 11-speed makes the most sense is on budget-friendly bikes or quality used models. You can often find great deals on 11-speed bikes since the industry has shifted to 12-speed as the new standard. If you're newer to the sport or riding a few times a month, 11-speed gives you reliable performance without paying for the latest tech.
The main downside is that high-end 11-speed parts are becoming harder to find as manufacturers focus on 12-speed. Replacement cassettes and chains are still available, but your upgrade options down the road are limited. If you think you'll want to upgrade to 12-speed later, it's usually cheaper to buy a 12-speed bike upfront rather than converting, since it requires new shifters, derailleur, cassette, and chain.