Same problem here. It is 2025 (and no one should use ckeditor anymore but we stuck on it through a third pary app).
I tried a bunch of different docTypes's, in both the top-level config.js and in core/config.js .
See that the docs are still there and the default value of this config is already html5.
https://ckeditor.com/docs/ckeditor4/latest/api/CKEDITOR_config.html#cfg-docType