code: @Restricted(DoNotUse.class)
      public HttpResponse doInstallStatus(StaplerRequest request) {
          try {
              String correlationId = request.getParameter("correlationId");
              Map<String,Object> response = new HashMap<>();
              response.put("state", Jenkins.getInstance().getInstallState().name());
              List<Map<String, String>> installStates = new ArrayList<>();
              response.put("jobs", installStates);
              List<UpdateCenterJob> jobCopy = getJobs();
  
              for (UpdateCenterJob job : jobCopy) {
                  if (job instanceof InstallationJob) {
                      UUID jobCorrelationId = job.getCorrelationId();
                      if (correlationId == null || (jobCorrelationId != null && correlationId.equals(jobCorrelationId.toString()))) {
                          InstallationJob installationJob = (InstallationJob) job;
                          Map<String, String> pluginInfo = new LinkedHashMap<>();
                          pluginInfo.put("name", installationJob.plugin.name);
                          pluginInfo.put("version", installationJob.plugin.version);
                          pluginInfo.put("title", installationJob.plugin.title);
                          pluginInfo.put("installStatus", installationJob.status.getType());
                          pluginInfo.put("requiresRestart", Boolean.toString(installationJob.status.requiresRestart()));
                          if (jobCorrelationId != null) {
                              pluginInfo.put("correlationId", jobCorrelationId.toString());
                          }
                          installStates.add(pluginInfo);
                      }
                  }
              }
              return HttpResponses.okJSON(JSONObject.fromObject(response));
          } catch (Exception e) {
              return HttpResponses.errorJSON(String.format("ERROR: %s", e.getMessage()));
          }
      }
docs: Get the current installation status of a plugin set.
  <p>
  Supports a "correlationId" request parameter if you only want to get the
  install status of a set of plugins requested for install through
  {@link PluginManager#doInstallPlugins(org.kohsuke.stapler.StaplerRequest)}.
  
  @return The current installation status of a plugin set.