Is HN actually filtering these, or they use utf8 in mysql which is famously not full utf8mb4?
But HN is 100% custom software, and built with it's own programming language as well, Arc (on Racket). AFAIK, HN still runs on files-as-a-db. You could check out the source here: http://arclanguage.org/install
"filesystem as a database" is not referring to database software that stores the data on disk but rather that the application is directly interacting with the filesystem. Imagine dumping a JSON document to disk, then reading it from disk, compared to storing a JSON string in a DB. Sure, they are both backed by the disk, but one is not "filesystem as a DB".