> JSON is the only popular language in the PLDB without comments.

Some folks actually tried addressing that limitation (and some others) with JSON5: https://json5.org/

Though sadly it never really caught on outside of a few niche projects. At the same time, however, I've seen some projects out there also use JSONC or another non-standard version that adds comments, since clearly people want them: https://komkom.github.io/jsonc/

Perhaps a more accurate statement will end up being "the only formally defined popular language".

Some redditers pointed out that (traditional) CSVs also don't have a mechanism for comments.