Product team have decided to remove this warning message and the change will be available in future TAS 4.0 release.
Before the release contains the change is available, if the user doesn't want to see the warning message on Apps Manager web UI, the following workaround can be used.
1) Check which application, apps-manager-js-blue or apps-manager-js-green, is started in org system / space system
2) Manually set env variable ENABLE_HTTP2 to true though HTTP2 is globally disabled
$ cf set-env apps-manager-js-blue ENABLE_HTTP2 true
$ cf restage apps-manager-js-blue