The International Agency for Research on Cancer identified a possible link between aspartame and a type of liver cancer called hepatocellular carcinoma.
This has been a thing since forever. I remember there being a big doobadoo about the shit in Diet Coke back in the 90's.
They showed it gave mice cancer.
The thing is that the study with the mice was seriously flawed. There's been more research since then, which is why we're getting this announcement now (even though the announcement itself is little more than "oh hey there might be something to this? We definitely need more research before we can know for sure.")