This is intentional. Application indicators have been deprecated for years. They have also been removed from GNOME in favor of better APIs. They were deprecated and not officially supported for two years in Loki, and have been fully removed for Juno.
There's some pretty good rationale on the GNOME design blog: https://blogs.gnome.org/aday/2017/08/31/status-icons-and-gnome/
@Djaler there isn't one for a concept of "status icons." Instead, there are several more tailored APIs that can be used depending on the needs of the app:
Gtk.Application for background processes. This is how apps in elementary OS continue operating in the background, i.e. Tootle checks for new posts, Mail notifies you of incoming email, Music continues playing in the background, etc.
FreeDesktop.org Desktop Actions for providing static quick actions, i.e. launching a specific part of an app from its icon in the Dock or Applications Menu.
LibUnity for dynamic quick actions, Dock progress bars, and badges in the Dock and Applications Menu. This is supported at least in elementary OS, Ubuntu Unity, Ubuntu 18.04+, and GNOME with Dash to Dock. I'm not sure about other DE support, but I imagine it is supported in KDE either out of the box or with an option. On elementary OS, even backgrounded apps that utilize these APIs will show up alongside other apps in the Dock.
Notifications for alerting people. Supported prety much everywhere.
MPRIS for audio applications to integrate with the system sound indicator. Supported in elementary OS, Ubuntu, GNOME, and several other DEs.
EDS for integration with the calendar indicator. Supported at least on elementary OS, Ubuntu, GNOME.
Maybe a elementary indicator using the new Cloud Providers API will be possible? A plugin for this API is currently being worked on in Files (see https://github.com/elementary/files/tree/plugin-cloudproviders)