KStatusNotifierItem new API need to be guarded with KNotifications version
Introduced in commit b3d86129.
My development setup is probably very unusual, so it's not surprising that the problem didn't show up before. What I do is most development on my main desktop system (which is dangerous, I know) and also I don't always keep everything at the same version (also dangerous, I know). So what must have happened was that I had updated kwindowsystem to 5.91 to investigate another problem, but didn't need to update knotifications which stayed at whatever its previously built version was. So konversation looked at the wrong version and tried to use the new knotifications API.
Unfortunately CI would not have picked up the problem as it builds everything from scratch every time.
Glad to be of assistance...