Description
In late 8.x releases, there's a bug in Fleet UI that causes 9.0.0
to appear as an option for agent upgrades. This option results in an error when selected, as upgrades across majors for Elastic Agent are not supported. e.g. agent can't go to 9.0 while Kibana and Fleet Server are on 8.x.
Resources
Agent upgrade is not selectable:

Fleet Server upgrade is selectable, with inconsistent UI state and error on submit:

Collaboration
The product team will provide the initial content.
Point of contact.
Main contact: @kpollich
Stakeholders: @elastic/fleet