What was the last Eps or Film or whatever with the 80s chipmunk design?

Alvin and the Chipmunks Meet The Wolfman (from the year 2000) would definitely be it.

Although, there's also Little Alvin and the Minimunks (from 2003) but that's more along the lines of live-action style except the chipmunks and the chipettes are in their puppet form and not cgi animated or even 2D animated. And their designs are a bit different because they're supposed to be portrayed as babies/toddlers in that film as well. However, that film is rather hard to find nowadays and isn't even for sale online or anywhere in real life because by my calculations, it would mostly be considered a lost film. If you happen to stumble across it somehow, you could consider yourself lucky. Other than that, it's very hard to find and it's rare for someone who happens to stumble across it.