I got annoyed enough to do something about it over the weekend. I built clearcalendar.app – you sign in, paste your calendar URL, filter out the events you don't want, and export a clean .ics file you can subscribe in any calendar app. Google Calendar, Apple Calendar, Outlook or anywhere you can subscribe calendars.
Would love to know if anyone else has had this problem – and if the tool actually works for your use case. Completely free right now.