I have no idea why patch notes are made available after the patch is applied these days. Patch notes should be available a week in advance at least, by that time its pretty obvious whats going to be added so why not post it right away.
There is a lot, and because a good few years is a bit vague I'll just go with some highlights.
We have 2 main story blockbusters (story modes) Black Heaven and Heroes of Maple. They both are a requirement to fight Lotus and Damien, the last two non-Arcane bosses.
The whole Black Mage 14 year long plot has come to its conclusion. At level 200 you get access to 5th job and the Arcane River, the highest level are and longest grind in the game and its currently the highest goal to achieve. The new bosses added from that plot are Lucid, Will, The Gloom, Verus Hilla, Captain Darknell and the Black Mage himself.
We get about 2 new characters every year, the most recent ones are Pathfinder (an explorer archer) and Ho-Young (a Grandis thief).
Aran and Evan got a revamp, Cygnus Knights got a revamp, Explorers had a revamp almost 5 years ago or so.
Some non-KMS areas were removed like Blackgate, Alishan and Twisted Aqua Road because nobody played them (I blame GMS).
Florina Beach got the axe (rip)
Masteria has gotten plenty of love, there are 2 Blockbusters (aka story modes) dedicated to it, Masteria Through Time and Monad: First Omen, both are kinda buggy but give some neat rewards (like BiS title and medal). Additionally the whole NLC and Phantom Forest got a full revamp and now begins at 160 onward, they have their own set of equipment (though its a bit grindy)
Root Abyss can be done as a single player now.
Gollux got some bad changes. Hellux is now a boss on the level of Lotus/Damien (aka very difficult) and give very little payoff, about 30ish coins. Superior Gollux stuff now costs 700 coins a piece.