I'm not aware of what you do and do not know, some I'm just going to be giving a basic rundown of all the major lore changes since then with the inclusion of a few champion lore changes. Side note, Summoner's Rift is no longer canon, and the game itself is no longer canon. The universe and lore are separate from the game itself. Anyway,
Ionia has largely stayed the same. They're still a peace loving culture, and compared to almost all other factions, haven't really changed so much as they have developed and expanded the lore. The same can be said about most of the champions, with a few exceptions. One of the few truly new additions is The Great Spirit of Ionia, which acts sort of like an embodying power of the people, representing their culture, but also connected to the land itself, and influences the people. Karma has been changed into being the "Karma" which is the physical incarnation of the Spirit of Ionia. Akali is no longer part of the Kinkou Order.
Zaun is now a city directly underneath Piltover, but overall, has remained as the dark toxic reflection of Piltover, filled with pollution and chemical clouds. Janna was changed from being a magical human into being a goddess/spirit that watches over the people of Zaun. Urgot has been relocated to Zaun, and no acts as a sort of warlord within the deepest parts of Zaun, and was betrayed by Noxus. Jinx has only become more deranged than before.
Demacia has been changed from an incredibly powerful nation rivaling Noxus, to a smaller, more insular kingdom, but still powerful in it's own right. They still uphold honor and justice, but a key part of Demacian culture in the new lore is their fear and hatred of magic. Lux has been changed into an unofficial undercover agent for Demacia, hiding her magic from her country and people, as well as her family. Galio is now a giant statue that Demacian's use when fighting magical enemies, and is only able to be "alive" when around a magical source.
Noxus was transformed from the "obvious bad guys" to a Roman-esque empire, conquering lands, not out of hatred or dark intention, but to thrive as a powerful nation. Swain is now the leader of Noxus, and is preparing for the return of Morderkaiser. Vladimir is now a near-immortal mage that has been around since the fall of Shurima, and although what he wants specifically is left a bit ambiguous, it's clear he seeks power, he's also a part of the Black Rose along with LeBlanc. Darius hasn't had his lore changed, so much as he has had it expanded, and is now a fully dimensional character, and even has children, though the specifics of his relationship with them is somewhat unclear.
In the Freljord, Lissandra has had her purpose completely redone. Instead of wanting to bring about the return of the Watchers, she's desperately trying to hide the fact that she almost brought the Void upon the world, and is the only thing keeping the Void from opening in the Freljord. Volibear and Anivia are now Demi-Gods, and worshipped by the tribes of the Freljord, and are even relatives.
Again, this is just the lore changes, and only the ones I cared enough to mention. If I were to have included all the lore updates as well, this list would be about 3 pages long. I highly recommend getting into the lore.
A good way to get into the lore is by starting off with a champion you like, and reading their lore, and expanding from there.
