Good grief. They've replaced a SQLite database with a text file that's loaded into an in-memory hash table because "adoption of HSTS is not very widespread yet" so "using any kind of off-the-shelf database to store this would be inefficient and overly complex." This required a patch that took over a year to review, during which time issues were raised with the text file parser they had to write from scratch. Loading the whole table into memory has clear DoS implications, so they're limiting the table to 1024 entries (making the use of a hash table rather silly), with an eviction strategy[1] that is going to favor older entries and effectively prevent newer entries from being added.
> Among the entries was one named track.nextuser.com
In order to be a supercookie they need more than one entry (since each entry only stores 1 bit of information). Do you see any other entries that look like they could be associated with this one?
To clear HSTS entries (which the "Clear recent history" UI does not delete), you can do:
I've been periodically monitoring this database for HSTS supercookies over the last couple years and have yet to see any in the wild.