I’ve checked my Google calendars and I saw the same thing.
Digging into the code suggests that this is disabled if I don’t have permission to edit the event.
In my case I can use this action on any event that I created myself but it is disabled for events that I have “imported”.
I.e. one of my folders in the Google calendar is a local organisation’s event calendar which I have added through the Google web interface.
It is essentially a web hosted ical file that Google will periodically check and then update the events.
At least by default these events seem to be “read-only”
You are right Krake, I changed the calendar settings to allow me to edit events, and now the option to Toggle Reminders On is available.
As an observation for other people looking for a similar issue, I shared the organizations read-only events with my personal gmail account. But I added reminders for said calendar in my personal account, then I get the reminders in Kontact/Korganizer, even if I don’t have the option to toggle it on/off from the app UI. (I did have to remove my account, and re-add to Korganizer for the changes to take effect)