Migrated from Sharkey to GoToSocial, because I wanted something more light-weight and less feature-rich (though I do have to say that Sharkey is excellent and has tons of features and control. I highly recommend it to somebody who really wants a lot from their Fediverse software). I probably sent out some unnecessary notifications by mistake while backloading my posts. Sorry about that.
I wrote this python script for doing the actual backloading. It's clunky, doesn't handle many common cases, breaks on mentions, and has tons of little flaws, but it did get the job done.
