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.
/* Copyright 2018 Mozilla Foundation
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
const compatibilityParams = Object.create(null);
if (typeof PDFJSDev === "undefined" || PDFJSDev.test("GENERIC")) {
  if (
    typeof PDFJSDev !== "undefined" &&
    PDFJSDev.test("LIB") &&
    typeof navigator === "undefined"
  ) {
    globalThis.navigator = Object.create(null);
  }
  const userAgent = navigator.userAgent || "";
  const platform = navigator.platform || "";
  const maxTouchPoints = navigator.maxTouchPoints || 1;