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.
pageDiv,
    pdfPage,
    annotationStorage = null,
    imageResourcesPath = "",
    renderForms = true,
    l10n = NullL10n,
    enableScripting = null,
    hasJSActionsPromise = null,
    mouseState = null,
    fieldObjectsPromise = null,