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.
if (!this.pdfDocument) {
      return false;
    }
    if (
      !(
        Number.isInteger(pageNumber) &&
        pageNumber > 0 &&
        pageNumber <= this.pagesCount
      )
    ) {
      console.error(`isPageCached: "${pageNumber}" is not a valid page.`);
      return false;
    }
    const pageView = this._pages[pageNumber - 1];
    return this.#buffer.has(pageView);
  }
  cleanup() {
    for (const pageView of this._pages) {
      if (pageView.renderingState !== RenderingStates.FINISHED) {
        pageView.reset();
      }
    }
  }
  /**
   * @private
   */
  _cancelRendering() {