SUMMARY

I've upgraded KDE Neon last weekend and most things went very smoothly.

It seems this release does no longer contain akonadi_kolab_resource but neither is its config migrated to an IMAP resource.

Had to manually create an IMAP account which lost all folder customizations (e.g. "contains mailinglist", "use specific identity, etc).

STEPS TO REPRODUCE
1.  Upgrade to KDE PIM 6.0.0 from 5.x version
2. Start KMail

OBSERVED RESULT

Entry for Kolab account is marked as "offline".
Log has many entries of akonadiserver not being able to contact the resource
/usr/bin/akonadi_kolab_resource does not exist

EXPECTED RESULT

Either akonadi_kolab_resource to run and handle its config or IMAP resource being in its place.
Ideally with the same collections.

In the worst case some info that the user has to re-create an IMAP "incoming" configuration manually.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: KDE Neon 6.0.0
(available in About System)
KDE Plasma Version: 6.0.0 (24.02.0)
KDE Frameworks Version: 6.0.0
Qt Version: 6.6.2

ADDITIONAL INFORMATION

Annoying side effect of the IMAP resource is that it shows all non-mail folders such as calendars.
I now have several folders with exactly the same name, just (luckily) in different locations in the tree.