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.
firstPageButton: document.getElementById("firstPage"),
      lastPageButton: document.getElementById("lastPage"),
      pageRotateCwButton: document.getElementById("pageRotateCw"),
      pageRotateCcwButton: document.getElementById("pageRotateCcw"),
      cursorSelectToolButton: document.getElementById("cursorSelectTool"),
      cursorHandToolButton: document.getElementById("cursorHandTool"),
      scrollPageButton: document.getElementById("scrollPage"),
      scrollVerticalButton: document.getElementById("scrollVertical"),
      scrollHorizontalButton: document.getElementById("scrollHorizontal"),
      scrollWrappedButton: document.getElementById("scrollWrapped"),
      spreadNoneButton: document.getElementById("spreadNone"),
      spreadOddButton: document.getElementById("spreadOdd"),
      spreadEvenButton: document.getElementById("spreadEven"),
      documentPropertiesButton: document.getElementById("documentProperties"),
    },
    sidebar: {
      
      outerContainer: document.getElementById("outerContainer"),
      sidebarContainer: document.getElementById("sidebarContainer"),
      toggleButton: document.getElementById("sidebarToggle"),
      
      thumbnailButton: document.getElementById("viewThumbnail"),
      outlineButton: document.getElementById("viewOutline"),
      attachmentsButton: document.getElementById("viewAttachments"),
      layersButton: document.getElementById("viewLayers"),
      
      thumbnailView: document.getElementById("thumbnailView"),
      outlineView: document.getElementById("outlineView"),
      attachmentsView: document.getElementById("attachmentsView"),
      layersView: document.getElementById("layersView"),