So what exactly is the New Plague ? I've played Point Lookout but i didn't really see much lore for it
That's because Van Buren was going to be centered around it. I recommend Retcon Raider videos if you want to get caught up on what the story with New Plague and the rest of the game was going to be, or read the leaked design documents yourself.
TL;DR is that this was basically Fallout COVID: It caused a major pandemic in the US that shut down everything, it caused people to get very sick and have strange side effects, it was spread by China(because of course it was) and the Chinese were trying to use it as a bio weapon in an attack on Hoover Dam before the war, that's what the two Chinese Stealth Suits in New Vegas are supposed to represent. In order to fight the worst side effects of the virus, and in turn to create a counter-bio weapon to sterilize and eliminate the Chinese, US tasked West Tek with looking into the FEV project, which ended up having quite different effects to what the government intended. The Super Mutants that FEV created DID resist the New Plague, as they also resisted radiation and other biological agents, not to mention their increased strength and physical durability, but the US military was more interested in FEV's potential for creating super soldiers, hence the experiments on human subjects in Mariposa. It can even be assumed that the different strain of FEV that Vault Tec was researching in Vault 87 was tasked with making dumb, obedient super soldiers with their own formula instead of focusing on the resistances like the original West Tek strain, hence the even dumber and even stronger muties of the East Coast. The founding fathers of Brotherhood of Steel were the first to find out what was really going on in Mariposa, and later Richard Grey found the vats as well, the rest is history.
BTW, ever wondered why the first target by the Chinese wasn't the White House but West Tek aka The Glow instead? This is why, if we follow added content from Fallout 2 Restoration Project as canon, the public actually knew about US Government creating bio weapons to sterilize the Chinese, which caused quite a stir among the public and especially the Chinese officials. We don't hear much about it because Chinese were our enemies after Anchorage and because there were so many other things going on that overshadowed it, like annexation of Canada, resource shortages, food riots and general public unrest. If you watch those Retcon Raider videos, you will also know that there were quite a few riots during the New Plague pandemic as well, it was a big deal in the Fallout world. The game that would have explained all that was never released, so I guess Bethesda wanted to confirm that the virus indeed exists, just not to what degree so that they could do something with it in the future.
wasnt there some debate from devs on the fev thing?
No, pretty much everyone who played the first game already knew this, since the first game isn't very big and The Glow would be one of the locations you would definitely want to explore, between some of the best loot being there and The Brotherhood(which you DEFINITELY want to join since they are the only ones with access to Power Armor) sending you there anyways. As I've written above, FEV is a big deal, and New Plague was the catalyst for it. West Tek was where all of the pre war problems plaguing New California can be traced from, between the facility being a prime target for the Chinese, the FEV being released into the air creating many mutations to concentrated FEV creating abominations like Super Mutants and Deathclaws, you can tell why Brotherhood feels so strongly about controlling the technology and keeping it away from layman hands, when they have to basically live next door to both Mariposa and The Glow. Trading all that away because the new crop of Fallout fans couldn't find an Oxhorn video explaining it to them for what is essentially a 50s parody of "Radiation makes ants big, haha!" is fucking stupid and exactly something that would appeal to the retarded Bethesda fanbase. This is why they are usually not taken seriously.
BTW Bethesda broke not only old lore but also their own lore many times, it's clear that they're either too stupid or too ignorant to spend 5 minutes looking things up on the wiki or reading the FOB. I wouldn't take their retcons too seriously, unless you really want to tell me that Jet was a pre war drug despite Fallout 2 hitting you over the head that it wasn't, or that Ghouls don't need air, food or water(Necropolis?) when they can just take magic science juice serum(the TV show introduced that, and yes Todd considers the TV show canon)