The composer.json of WikiSEO has a name of mediawiki/wiki-seo but that's not correct (it doesn't exist). The name should be octfx/wiki-seo.
The other issue is that that packagist record points to https://github.com/octfx/wiki-seo as the source, rather than https://gerrit.wikimedia.org/g/mediawiki/extensions/WikiSEO
There seems to be an issue with version numbers too. There's a 2.6.6 version in the Gerrit composer.json, but that probably doesn't need to be there because Composer should just read from the Git tags to find the versions. Getting rid of it would make things easier to keep up to date too.