Push - Because of a new announcement by the GNOME team.
The GNOME team announced that the tray bar will be deprecated by GNOME 3.26. This means that status icons will not be shown by default starting with that version. You can read the whole topic here: https://blogs.gnome.org/aday/2017/08/31/status-icons-and-gnome/
There is also a new library called libcloudprovider which allows to integrate cloud services a bit more into the system.
This topic shows the guidelines for developers who want to make sure their program is GNOME-compatible: https://wiki.gnome.org/Initiatives/StatusIconMigration/Guidelines
It states - for example like I mentioned in my last post here:
Ensure that all application functionality can be accessed from the application's windows, and that no actions or options are only in the status icon menu.
I hope you think about what that means to the Seafile client. Ubuntu will be based on GNOME in the future. It's also Debian's default desktop and the same goes for Fedora.
Please have a look at the status icon migration guidelines for GNOME (second hyperlink above) @Jonathan.