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.
return this.#annotationEditorMode;
  }
  /**
   * @param {number} mode - AnnotationEditor mode (None, FreeText, Ink, ...)
   */
  set annotationEditorMode(mode) {
    if (!this.#annotationEditorUIManager) {
      throw new Error(`The AnnotationEditor is not enabled.`);
    }
    if (this.#annotationEditorMode === mode) {
      return; 
    }
    if (!isValidAnnotationEditorMode(mode)) {
      throw new Error(`Invalid AnnotationEditor mode: ${mode}`);
    }
    this.#annotationEditorMode = mode;
    this.eventBus.dispatch("annotationeditormodechanged", {
      source: this,
      mode,
    });
    this.#annotationEditorUIManager.updateMode(mode);
  }
}
export { BaseViewer, PagesCountLimit, PDFPageViewBuffer };