Thomas, Graham: there is a discussion about a KDE5 port of the
syncevolution akonadi plugin, which currently supports KDE4 only. KDE4
will not be part of the next Debian Stable release (Buster).
I have prepared a syncevolution package which compiles with KDE5.
Kwallet support is disabled for now, but the akonadi backend
If anybody wants to test this, just reply to this mail. If tests go
well, I will try to finish the KDE5 port. Otherwise I'll disable the
akonadi plugin for Buster.