Peeebs (on Twitter or the Arthur Slums forums) actually has the schedule. S25 is the last season (with it and S24 noticeably only having 4 episodes)
I really doubt the ending itself will be impactful...but it'll be a much needed rest for Arthur
Actually, according to the Arthur Wiki, Season 24 will have just three episodes (technically, two, since the second episode of that season is a remastered "The Great McGrady"). Season 25 will have four episodes, the last segment being appropriately titled "All Grown Up". There will be no two-part episode to close out the series, strangely enough, but if I were to guess, the final segment might focus on the characters as mature adults and teenagers, and imagine what kind of life Arthur and his wife, as adults, reminiscing about their childhood. Other possible items could be high school or college graduation, and/or marriage. Nothing morbid or sad, implying that the series is expected to go out on a good note.
I am thinking the reason for Season 25 going ahead may be due to contractual obligations, and the production company decided to end production after that. The series may not be cancelled, it's just ending. The cast knew it was coming, nevertheless.
I am thinking the newer episodes nowadays may end up being a rehashing of older episodes, but modified - if slightly. Cost-cutting at its finest.