Allow Fleet to be disable-able in 8.x

[JAndritsch]

Describe the feature:

A recent change made it so that all plugins must opt-in to being disable-able: #89584. Fleet is one of several plugins that do not currently support this behavior. Prior to this change, one could disable access to Fleet via xpack.fleet.enabled: false. That is now deprecated in 7.x and incompatible in 8.x.

An alternative way to "disable" plugins in Kibana is to simply delete them from the x-pack/plugins folder. That does not work for Fleet, however, as it seems other plugins depend on its source code. The changes referenced in the above issue make it so the Fleet UI and its functionality cannot be hidden globally for all users.

Is it possible to have Fleet opt-in to this behavior so that it can be disabled once again?

Describe a specific use case for the feature:

The environments where I deploy the Elastic Stack do not support the functionality offered by the Fleet plugin, so I disable it to prevent users from interacting with UI that does not work. Fleet is not the only plugin that I disable in 7.x.

The users and roles in these environments are not managed by me. I cannot control how user permissions are set in those environments and cannot use Kibana Feature privileges to globally disable the plugin for everyone in all environments.

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[kpollich]

Hey all - apologies for the very late response here. Doing some issue clean up and came across this elsewhere.

Disabling Fleet post 8.x is not something we intend on supporting. Fleet is consumed programmatically by too many other plugins across our security and observability solutions that disabling Fleet results in a severely degraded or outright broken experience in those other plugins. For example, APM and Synthetics both rely on Fleet's policy experience for managing their respective configuration objects, and without Fleet's policy API's or related React components available those plugins simply can't function.

In 8.16, we introduced "solution oriented navigation" to Kibana, where the Kibana UI can be tailored to a specific solution. These solution oriented UI's place Fleet in a more deeply nested navigation menu under "management" rather than placing it directly in the sidebar. This might be of some interest to make Fleet a bit less prominent in your Kibana UI: https://www.elastic.co/guide/en/kibana/8.16/whats-new.html#_solution_oriented_navigation.

In 9.0, we'll also ship a large set of changes to allow Fleet access to be limited more granularly through Kibana roles and spaces. See public issue here for context: #175831

With all of this in mind, I'm closing this issue as won't fix.