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.
value: typeof PDFJSDev !== "undefined" && PDFJSDev.test("TESTING"),
    kind: OptionKind.VIEWER + OptionKind.PREFERENCE,
  },
  annotationMode: {
    
    value: 2,
    kind: OptionKind.VIEWER + OptionKind.PREFERENCE,
  },
  cursorToolOnLoad: {
    
    value: 0,
    kind: OptionKind.VIEWER + OptionKind.PREFERENCE,
  },
  defaultZoomValue: {
    
    value: "",
    kind: OptionKind.VIEWER + OptionKind.PREFERENCE,
  },
  disableHistory: {
    
    value: false,
    kind: OptionKind.VIEWER,
  },
  disablePageLabels: {
    
    value: false,
    kind: OptionKind.VIEWER + OptionKind.PREFERENCE,
  },
  enablePermissions: {