Below are some comparisons of the switch between using traditional cels to digital for the art of InuYasha. I took these screenshots from the Viz DVD Box Sets for the Fourth Season & Fifth Season. The left is traditional, right is digital. Click / tap for a larger image.
The change to the series being completely animated digitally happens on episode 99. All episodes before that still used cels. In my opinion, the change is fairly noticeable. The quality of art & animation varies throughout this series based on the episode, but there is a definite change in the color and the look of the show once episode 99 comes along. The episodes up through episode 98 are clearly a bit dimmer & grainier, while the episodes done digitally are more vibrant but detail tends to look more flat. It's interesting to note that the 3rd opening & 4th ending and onward were mostly done digitally, even though at that point the show was still being animated using cels. The first movie also has been done at least partly on cels, with some use of CG, and the following movies were done digitally. A quick google search for "InuYasha cels" yields a lot of scans of cels not only from the series but the 1st movie as well. They are even being sold on eBay & etsy, sites like WorthPoint document this as well (example). There are also some sites made by collectors of anime cels out there, tarakatsuki Cels has a lot of examples, including cels from the 1st movie. Maiken2051 also has a great collection, there are even cels from some of the last traditionally drawn episodes here (Ayame from ep83/84) & Enju (ep.92). Maiken2051 also shows that some art of the Band of Seven arc was done traditionally, interestingly enough. Here is a cel from the 5th opening featuring Bankotsu "I was really happy to get this OP cel of Bankotsu, even though it doesn't show his whole face. He's normally CGI only, curse it, so this is about all there is." There are also sketches of Jakotsu and even Hakudoshi & the infant from later in the series, but no more cels.
*Update 5/14/2020: Here is another site with cels, AlexGundam's Anime Cel Showcase, featuring several cel scans from InuYasha. On this page there is a cel from episode 98 "Kikyo and Kagome: Alone in the Cave". Quote from the site creator "This shot occurs while Kagome is in the cave protecting Kikyo. From the last episode of Inuyasha to use cel animation." There are also cels on Sean's Anime, featuring several cels from the "4th Season" of InuYasha and the 1st Movie.
The change to the series being completely animated digitally happens on episode 99. All episodes before that still used cels. In my opinion, the change is fairly noticeable. The quality of art & animation varies throughout this series based on the episode, but there is a definite change in the color and the look of the show once episode 99 comes along. The episodes up through episode 98 are clearly a bit dimmer & grainier, while the episodes done digitally are more vibrant but detail tends to look more flat. It's interesting to note that the 3rd opening & 4th ending and onward were mostly done digitally, even though at that point the show was still being animated using cels. The first movie also has been done at least partly on cels, with some use of CG, and the following movies were done digitally. A quick google search for "InuYasha cels" yields a lot of scans of cels not only from the series but the 1st movie as well. They are even being sold on eBay & etsy, sites like WorthPoint document this as well (example). There are also some sites made by collectors of anime cels out there, tarakatsuki Cels has a lot of examples, including cels from the 1st movie. Maiken2051 also has a great collection, there are even cels from some of the last traditionally drawn episodes here (Ayame from ep83/84) & Enju (ep.92). Maiken2051 also shows that some art of the Band of Seven arc was done traditionally, interestingly enough. Here is a cel from the 5th opening featuring Bankotsu "I was really happy to get this OP cel of Bankotsu, even though it doesn't show his whole face. He's normally CGI only, curse it, so this is about all there is." There are also sketches of Jakotsu and even Hakudoshi & the infant from later in the series, but no more cels.
*Update 5/14/2020: Here is another site with cels, AlexGundam's Anime Cel Showcase, featuring several cel scans from InuYasha. On this page there is a cel from episode 98 "Kikyo and Kagome: Alone in the Cave". Quote from the site creator "This shot occurs while Kagome is in the cave protecting Kikyo. From the last episode of Inuyasha to use cel animation." There are also cels on Sean's Anime, featuring several cels from the "4th Season" of InuYasha and the 1st Movie.