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.
kind: OptionKind.API + OptionKind.PREFERENCE,
  },
  disableRange: {
    
    value: false,
    kind: OptionKind.API + OptionKind.PREFERENCE,
  },
  disableStream: {
    
    value: false,
    kind: OptionKind.API + OptionKind.PREFERENCE,
  },
  docBaseUrl: {
    
    value: "",
    kind: OptionKind.API,
  },
  enableXfa: {
    
    value: true,
    kind: OptionKind.API + OptionKind.PREFERENCE,
  },
  fontExtraProperties: {
    
    value: false,
    kind: OptionKind.API,
  },
  isEvalSupported: {
    
    value: true,