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.
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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