The show states that "Simu Liu will not sign any Master of Kung Fu comics or other comics deemed offensive. All autographs from Simu will be signed in English only." Shang-Chi first appeared in the Master Of Kung-Fu comic books that were published by Marvel Comics from the seventies through to the eighties, 125 issues all in, a considerable run. Shang Chi first appeared in Special Marvel Edition #15 in 1973, created by Steve Englehart and Jim Starlin, and the title was changed to The Hands of Shang-Chi: Master of Kung Fu two issues later. The vast majority of Shang Chi appearances were in those issues, and Simu Liu has made it clear he won't sign any of them.
Why might they be considered offensive? Well, the phrase bandied about these days is that the comics are "of their time". While they have an East Asian protagonist, and admired creative runs by Doug Moench, Paul Gulacy and Mike Zeck, the comic books are steeped in "Yellow Peril" tropes of the day, with Shang-Chi's father actually being the embodiment of such, Fu Manchu, when Marvel had the licensed rights to the character. Indeed, Jim Starlin left the series after he was made aware by fellow comic book creator Larry Hama of the racist nature of the Fu Manchu novels.
The current Marvel comic books featuring Shang-Chi, now simply called Shang-Chi, written by Gene Luen Yang and drawn by Dike Ruan and Marcus To are more acceptable to be signed by Simu Liu. And here's the thing, it's not like he actually owes anyone a signature, anyone is clearly free to set boundaries over what they will or will not sign, this way people know in advance to avoid disappointment on the day.