As a past Evernote "junkie" I can understand where this request from OP is coming from. In fact, I first switched to Joplin, then went back to Evernote once more (manually migrating the notes I had taken in Joplin during those weeks...). Only a few weeks later again did I decide that Evernote (10...) is indeed no longer usable and too laggy for about any real usage, coming back to Joplin for good.
Second, given the vast number of note-taking apps that can read ENEX, it is a de facto standard. However, I assume, it is more of a strategic question if you want to support ENEX export or not. Maybe it would be better to ensure Joplin and JEX become the de facto standard? Go tell Evernote if they want their users back (I am by far not the only one making the Evernote 10 exodus...), they will need to write a JEX importer?
I.e., leaving Joplin for Evernote because you cannot go back to Evernote is, well, let me say. peculiar, all the more considering how unusable Evernote has become and how deep it has sunk (after being considered by me and many others the most important app to have on your computer!)
Whatever, and in any case, for those in the want, there is a feature request for exactly this: [Feature request] Export to ENEX