Do LED lights require a special dimmer switch?
Do LED’s require special dimmers, It depends, with 9 lights there is probably enough of a load to operate properly (especially if they have worked fine for 2 years). If there was no glowing when turned off and flicker when new the lights are probably failing.
All the lights are in parallel so if they are all of a sudden operating at different light levels they may have been cheaply made and the drivers are failing.
It is possible your dimmer is not operating properly but with different light levels at the lights this points to driver failures
If you bypass the switch (power off) wire nut line to load and power on if the fixtures are dim that proves they are failing.
I suggest only purchasing DLC certified lamps and fixtures.
DLC stands for Design Light Consortium, DLC required a 5 year warranty so they might be slightly more expensive but that is better than them failing not long after warranty. Also get your lights and fixtures from reputable dealers like 1000 bulbs. Com or LEDMYPlace.com (I get most of my lights and fixtures for work and home here , no affiliation other than a satisfied customer and the very few failures I have had they promptly replaced.