The update screen in the Web UI shows which apps are compatible/not compatible when offering an upgrade. I believe it says Apps missing compatible version or something like that.
So, yes, it’s expected behavior if you upgrade Server and an app doesn’t have a published compatible version.
Presumably there will be a new GF release that is v29 compatible in the relatively near future. Also, v28 is still well supported and will remain so for another 8 months (it will continue to receive security/bug fixes monthly like usual).