Please remove this line and the indentation after.

This pulls in KastenCore automatically I assume? -> Then don't specifically search for it.

Similarly

Similarly

Remove

Remove

Duplicate

This file should have proper indentation. Did sth go wrong with desktoptojson?

Fix indent

Nitpick: Use new-style connect

As you said: This file should probably be killed?

QVariant::fromValue(...)?

TODO note can be removed with this work :)

Almost all of the below can be removed.
Actually the only lib which needs to be searched for explicitely is OktetaKastenControllers.
Its config cmake dependency chain requires and imports all the others already (unless something else regressed, poke me if).
So this should be enough for this whole file (untested):

find_package( OktetaKastenControllers 0.3.1)
set_package_properties(OktetaKastenControllers PROPERTIES
        PURPOSE "Required for building Okteta KDevelop plugin."
        URL "https://kde.org/"
        TYPE OPTIONAL)

if (OktetaKastenControllers_FOUND)
    add_subdirectory(okteta)
endif()

If you want to be a little bit more explicit, you could require

find_package( OktetaGui )
find_package( KastenControllers )
find_package( OktetaKastenControllers )

to reflect the three logical groups here (OktetaCore/Gui, KastenCore/Gui/Controllers, OktetaKastenCore/Gui/Controllers). But then it does not really add value, so you could leave that out.

Why remove these? The subdir should be only entered to build the plugin if the optional deps are found, no?
It rather needs fixing to use the proper _FOUND vars (matching the lower/uppercasing of the package name(s)), and just if (OktetaKastenControllers_FOUND) should be enough here

To be removed now.

Just OktetaKastenControllers should be enough. If you prefer explicit listing of all libs, the complete list is

OktetaKastenControllers
OktetaKastenGui
OktetaKastenCore
KastenControllers
KastenGui
KastenCore
OktetaGui
OktetaCore

Can be removed now as well, give there is no more desktop file generated and used.

If you prefer, CamelCase includes should now be possible for all classes, so e.g. here

#include <Kasten/Okteta/ByteArrayView>

and similar for all other includes.

But plain header includes like you kept using for now work of course as well. Just in case you preferred them, for consistency :)

Turned out that find_package( KastenControllers ) is also required.

This same here. Also KastenControllers is required. (linking errors)

I don't want to fix this. Maybe it would be prettier, but the required work is not worth it.

I actually used some find&replace with RegExp to generate the previous one. New one is generated with dektoptojson. (the lack of indentation was based on the link)