If you don't get an answer here you might try the privacy communities. There may be a way to convert one of the other calendars to Proton.
Edit : This might not help if you have completely disabled your Google calendar or don't want to involve Google at all but I was able to add Kansas City's schedule by going here and adding the calendar to my Google account and then adding the calendar in Proton by adding a calendar and using their Google import. It's kind of lame, but I didn't have to have Google calendar installed to do it.
Hmmm. I'll admit that I don't want to give this permission, even though it's not a calendar I use:
See, edit, share, and permanently delete all the calendars you can access using Google Calendar
Emphasis mine.
This is good to know, though. Thank you!
Also, there are few enough games that I could probably just input them manually rather than worrying about including an external calendar. I should probably just do that.
I wouldn't use Google Calendar. Instead, you can grab the ICS Yahoo sports calendars as a read-only subscription. I'm sure Proton Calendar offers it and it doesn't require connecting to Google or exposing anything funny. I don't know if Yahoo offers any public documentation on it but here's an example feed, you can tweak it by just searching for your team on Yahoo Sports and adding /ical.ics to the URL (or just guessing):