This is about an icon name. Apps don't (shouldn't) "check the value".

OK for deprecating Incremental. A full revert (at KF6 time) of commit 9ac82e27ad0322e444c16 looks tricky though :-)
I searched the git history, and I can't see where we ever used FindIncremental... [I searched kdelibs, since I'm pretty sure this was added for KHTML]

Nice work! Found 2 bugs though.

This makes runService and runApplication almost the same, right?

kfileitemtest still passes?

How about we do this more like in KMail ?

Yes, I like it too.

This code only seems to do append() and contains(), a QVector (contiguous area of memory) is better than a std::list (individual nodes spread all over the memory, more cache lines to load)

I'm confused. The units singleton shouldn't be exported but Kirigami.Units is fine? What's the difference?

Well, I'm not the KConfig maintainer, mdawson is :-)

I don't really understand the relation to ksmserver, surely kioslaves are not part of session management.

feverfew: you're probably looking at master while this is a patch for the 19.12 branch.
See https://phabricator.kde.org/D26358 which happened in master.

Both are supported, but indeed json is preferred over installed .protocol files.