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.
if (pageMode && sidebarView === SidebarView.UNKNOWN) {
            sidebarView = apiPageModeToSidebarView(pageMode);
          }
          if (
            pageLayout &&
            scrollMode === ScrollMode.UNKNOWN &&
            spreadMode === SpreadMode.UNKNOWN
          ) {
            const modes = apiPageLayoutToViewerModes(pageLayout);
            
            
            
            spreadMode = modes.spreadMode;
          }
          this.setInitialView(hash, {
            rotation,
            sidebarView,
            scrollMode,
            spreadMode,
          });
          this.eventBus.dispatch("documentinit", { source: this });
          
          
          if (!this.isViewerEmbedded) {
            pdfViewer.focus();
          }