It says, right on the mobile mode interstitial, that it "requires the latest mobile/tablet hardware." And yet, you seem surprised when it doesn't work on older hardware?
I was going to disagree because the same countries that really like web games are often the ones using older handsets, but Apple themselves say only 8% of handsets are on iOS < v10: https://developer.apple.com/support/app-store/
Even at it's peak apple only had 23% market share. 1 in 10 out of 23% is 2.3% and not everyone plays this game on a phone so the percentage is even lower in reality.
Probably not worth supporting. :) Adding new features and figuring out ways to keep people playing might be much more worth your time.