The plain text alternative is generated by soundasleep/html2text 2.1.0, licensed MIT (GPL-compatible). Since 1.14.0 it is installed by Composer into vendor/ (composer.json requires ^2.1) and its autoloader is loaded on the first HTML email of a request, unless another plugin has already loaded Soundasleep\Html2Text. Before 1.14.0 the plugin carried a hand-copied, locally patched 0.5.0 in includes/, under the EPL 1.0 / LGPL.
ignore_errors on, because upstream’s default path lets libxml emit a PHP warning for every HTML5 tag or stray end tag in an email body.char_set; without it non-ASCII text in emails with no charset meta tag came out garbled.vendor/ is gitignored and wiped by a zip install. Until Composer is re-run, emails go out HTML-only and each one logs that vendor/autoload.php is missing.