I was wondering the same, but I'm not sure if it would make a major change in the graphs. MySQL and MariaDB have both been unpopular on Hacker News for many years. Submissions on either topic rarely get much traction, which then leads to fewer submissions.
> MySQL trending downwards would make sense as people upgrade and switch over.
No, most large MySQL users are still using MySQL; there hasn't been a widespread migration to MariaDB. They're both actively developed and have grown in slightly different directions. Among corporations, MySQL's usage still far outstrips MariaDB by a significant degree. Lately MariaDB has better product velocity though, and their commercial enterprise finally seems to have stable footing.
I don't think I even knew I was running MariaDB at first, or perhaps more as a side note that I saw it dropping in mariadb when I apt installed mysql. If you upgraded Debian some time ago, I'm pretty certain you were automatically migrated, so anyone running that (or, presumably, one of the derivatives like Ubuntu) would have migrated knowingly or unknowingly, hence my assumption
And then there's all the users of managed cloud database offerings (RDS, Cloud SQL, etc) who definitely don't accidentally switch database vendors in that manner. Google Cloud doesn't even offer managed MariaDB, and Azure is retiring their managed MariaDB product.
Also keep in mind MariaDB hasn't been fully drop-in compatible with MySQL for over a decade. They've increasingly diverged in features and minor syntax differences over time.
Just to be clear, I'm not bashing MariaDB, I quite like it as a database. But there's a lot of misconceptions about the relative usage levels of MariaDB vs MySQL among FOSS circles.