I have not watched the Pokémon anime since Battle Frontier, but I saw a snippet of a much newer episode in a region that is most definitely not Johto, that featured Claire, the Dragon gym leader from the Johto region.
This led me to realize that it's not impossible for the anime producers to bring back characters from the very distant past.
So I was thinking, does Ash ever re-encounter his Butterfree in an episode after Butterfree's Goodbye?
A simple answer is no. We don't see Butterfree and Ash ever meeting again after their farewell in Bye-Bye Butterfree:
A scene just before butterfree's departure
As mentioned on the Bulbapedia (and partially in the comments):
Ash's Butterfree has since appeared in the tenth opening for the anime, Spurt!, along with his mate, the pink Butterfree.
However, he has not returned in any episodes since his last appearance. Ash has, however, flashed back to having him several times, such as in The Thief That Keeps on Thieving!, A Trainer and Child Reunion!, and Butterfree and Me!.
Yes.
In the final episode of Pokémon Journeys, "Satoshi and Go! Embark on a New Journey!!", aired in Japan on 16 December 2022, Ash reunites with both Butterfree and its mate:
For context, this is exactly 1,200 episodes after Butterfree was initially released.
At the time this of post, the answer is no. However, there might be a possibility of Butterfree returning in Pokemon Master Journeys.
I'm assuming it died. If it's anything like a real-life butterfly or insects in general which have short life spans, real butterflies (depending on the type) live from a few weeks to a year at most. Considering how in Pokemon insect types evolve the fastest and are found in bulk it should correlate they die the youngest and lay many eggs to compensate like real life insects as well.
