Summary

Currently, pdf output can export fonts in either type 3 or type 42 formats. Type 3 fonts (the default) are quite limited and e.g. do not provide kerning information, so we need to manually include that (#18282) when converting from a TrueType (~type 42) font. They also appear to be rejected by some publishers (https://phyletica.org/matplotlib-fonts/,https://tex.stackexchange.com/a/659918).
Type-42 support in Matplotlib has been full-featured, AFAICT, since we started to support type-42 subsetting via fontTools.

Proposed fix

Consider deprecating support for embedding fonts as type3 in pdf, or at least switch the default to type42.