github.com/weskerfoot/Del now supports deleting and fully archiving all of your messenger conversations :) Still needs a bit of fine tuning, but it's working extremely well so far. It potentially could be used to scrub and archive conversations on a timer as well.
Conversation
And also if you aren't familiar with it, it also supports deleting all of your wall posts, and unliking all of the things you've liked (and archiving that too)
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Oh, and it also supports filtering by year, and it will store every message with a timestamp and user ID, along with a list of extracted image links