Using New Outlook on Windows for personal email, calendar, and contact management
Found a workaround for this problem:
Try subscribing the tripit.ics calendar to google calendars. Go under "add a calendar" and "subscribe to a URL" Add it as a public calendar
Once you have added the calendar, select it and select "settings" under the ... menu.
If you cross down your will see a URL listing for this calendar in "ical" format.
I was successful in subscribing to this calendar feed in my MS 365 outlook calendar.
Hope this helps