In the past I’ve experimented with archiving old WordPress sites by converting them to HTML files and hosting them at archival URLs. The plugin I used to use is no longer in the WordPress repository. The author has WP2Static hosted on GitHub and a note on why the move and where the project is headed. Also here:
I did try using Simply Static but initial experiments failed and even though I thought I could probably make it work, I decided to go with WP2Static version 6.6.21.