We could use:
https://github.com/mozilla/pdf.js/blob/c3d08580627471432a6ad0b2fdb948df64126afa/src/display/api.js#L929

and look at bit 4.

The following pdf:
https://orimi.com/pdf-test.pdf
is not editable in neither Acrobat nor Edge nor Foxit and nor Okular.
editorNoneButton,
    editorFreeTextButton,
    editorInkButton,
  }) {
    this.eventBus._on("annotationeditormodechanged", evt => {
      const editorButtons = [
        [AnnotationEditorType.NONE, editorNoneButton],
        [AnnotationEditorType.FREETEXT, editorFreeTextButton],
        [AnnotationEditorType.INK, editorInkButton],
      ];
      for (const [mode, button] of editorButtons) {
        const checked = mode === evt.mode;
        button.classList.toggle("toggled", checked);
        button.setAttribute("aria-checked", checked);
      }
    });
  }
  _updateUIState(resetNumPages = false) {