once you improve your tactic rating you will get harder puzzles, if you have 2000 rating on it you will more likely get from 1800-2200 puzzles if you are 2500 rating you could get 2100+ since there isn't a puzzle rated 2600+ that i know, if there is, i would like someone could share us a link of it.
Yes, the Puzzles at the same ELO Lever got harder in the last time. Just look at the Youtube Video"Stream recap - Solving puzzles (30-11-2017)" from TricksOnly. The first puzzles there had a rating around 2100 a year ago but only have a rating around 1900 now.
Actually I'm experiencing a totally different phenomenon now, maybe because I am doing puzzle numbered above 100000 due to the sheer number of puzzles I've done. I was around 2000-2200 for few months where some 2600 puzzles were as hard as 9 move deep sequence, and suddenly I jump to 2500-2600, where I see for the first time some puzzles around 2700-2800, but are just puzzle that finish in 2 moves. I'm not fully sure I am able attribute my sudden rise in puzzle rating to an actual improvement. Some 1800 rated puzzles were actually harder to calculate non-engine defense lines than some 2100 rated puzzles, but was very intuitive.
Otherwise... it may just be that I've become a tactical monster (only when opponent blunders, since lichess puzzles aren't really generated through inaccuracies or mistakes).