Here we see the file(1) command knows a file has very long lines.
$ file www.youtube.com.har
www.youtube.com.har: UTF-8 Unicode text, with very long lines
$ wc www.youtube.com.har
   45982   330703 14075335 www.youtube.com.har

Alas, even in my https://www.jidanni.org/comp/configuration/.emacs with
(global-so-long-mode 1)
(setq so-long-action 'so-long-minor-mode)
this certain file does not trigger so-long mode.

Where to get such a file to test?
Real simple. In Chrome Developer Tools network tab, visit
https://www.youtube.com/c/jidanni2/playlists?view=1 and right click
"Save all as HAR with content."