For American/Canadian cartoons i'd say the cutoff is somewhere around the late 90s, but no further than that.
Dragon Tales (1999) was one of the last cartoons to be released in the 90s, but you'd be forgiven for thinking it came out around the same time Arthur (1996) did; over half of the series (S1; 40 episodes) was cel-animated. Season 1 also has a hazy, fever dream atmosphere. The designs of the dragon characters feel sort of jarring to be brand new in 1999 (they feel like designs from 1993-1996). As a result, it felt sort of dated even in 1999 (as most new cartoons released in 1999-2000, with a few notable exceptions started out digitally animated); the optimistic era of Y2K does sort of make Dragon Tales feel less out-of-place debuting as a brand new cel-animated cartoon, but the entire 90s decade was pretty optimistic in general (of course, not entirely peaceful, but relatively good for America overall).
Meanwhile another educational cartoon, Dora The Explorer (2000) came out just one year later, but does not feel old-school at all. It's tone is entirely new-school, the characters, their designs and personalities, are not dated at all, the show also has tons of preschool cartoon cliches too. I'd argue that Dora is the first "modern" educational cartoon, as it popularized many cliched preschool cartoon tropes you'd find in educational cartoons from years if not decades afterwards
Compared to Dragon Tales, the next original IP from PBS (not based on a book or comic or something else), Cyberchase (2002) is also not really "old-school" (although since the show's depiction of the internet is from an early 2000s perspective, it is becoming quite dated as time goes on).
And even in 1999, while there were many more digitally animated new cartoons (Rocket Power, Courage The Cowardly Dog and Family Guy), there were still plenty of cel-animated cartoons that premiered brand new in 1999 (SpongeBob SquarePants, Mike Lu & Og, and most notably, Ed Edd n Eddy).
Most cartoons premiered in 2000 or later were digital from the start. There were a handful of exceptions (Whatever Happened To Robot Jones in 2002 and Buzz Lightyear of Star Command in 2000), but they were the exception rather than the rule. The latter actually only had about half of it's episodes cel-animated, with the other half done digitally.
