I can successfully add my TripIt calendar feed to Outlook on my personal computer which is subscribed to Microsoft 365 (Outlook version 2311 - Build 17029.20068 Click-to-Run).
However, I can not add the same calendar feed to Outlook on my work computer, which is subscribed to Microsoft 365 Apps for enterprise (Outlook version 2311 - Build 17029.20068 Click-to-Run). Same version of Outlook but doesn't allow me to add the ics URL (error message says "Sorry, something went wrong. You may want to try again.")
So maybe this is why TripIt claims they don't support "Outlook 2013 (and later)"? Because it works inconsistently? It still seems like a Microsoft problem to me (or at least they should be willing to work with SAP's TripIt development team to provide guidance on how to fix it). Calendar sync works fine with Google Calendar by the way but it's irritating to have to check and monitor multiple calendars. I just want to have my trips show up on my work calendar.