The Google Calendar linked on https://community.kde.org/Schedules/Plasma_6 seem to no longer be updated. This project periodically checks that page and converts it into an ICS calendar so you (and I) can subscribe to the calendar without having to check that page.
Add this to your calendar: /plasma6.ics
Source code: https://gitlab.com/kisaragi-hiu/ics-for-plasma-release-schedule/
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