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.
const { eventBus, _boundEvents } = this;
    _boundEvents.windowResize = () => {
      eventBus.dispatch("resize", { source: window });
    };
    _boundEvents.windowHashChange = () => {
      eventBus.dispatch("hashchange", {
        source: window,
        hash: document.location.hash.substring(1),
      });