I was looking for a post I knew I'd written, and finally found it on Medium. I don't know why I'd never moved it somewhere else, and this bugged me, so I started downloading posts one-by-one. I only have around 150 posts on Medium, but that's still a lot of work. Turns out I could export my entire Medium account using an option in the Medium control panel, so I did. The only drawback is that image links in the downloaded files reference the Medium CDN, meaning they are not complete archives. I'll probably still manually download my favorites, using the terrific SingleFile extension, so the archives are completely self-contained HTML documents with no external dependencies.