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.
pages: [],
    };
    if (this._onBeforeDraw) {
      this.eventBus._off("pagerender", this._onBeforeDraw);
      this._onBeforeDraw = null;
    }
    if (this._onAfterDraw) {
      this.eventBus._off("pagerendered", this._onAfterDraw);
      this._onAfterDraw = null;
    }
    if (this.#onVisibilityChange) {
      document.removeEventListener(
        "visibilitychange",
        this.#onVisibilityChange
      );
      this.#onVisibilityChange = null;
    }
    
    this.viewer.textContent = "";
    
    this._updateScrollMode();
    this.viewer.removeAttribute("lang");
    
    this.viewer.classList.remove(ENABLE_PERMISSIONS_CLASS);
    if (this.#previousAnnotationMode !== null) {
      this.#annotationMode = this.#previousAnnotationMode;
      this.#previousAnnotationMode = null;
    }
  }
  #ensurePageViewVisible() {