TripIt does what you want, and more. (Imports flight confirmation emails automatically, and gives you an iCal feed you can subscribe to in your calendar)
Interestingly, I had a brief look for open source alternatives and found that KDE Itinerary[0] may do a similar thing, and that the same code has been re-used in a Nextcloud Mail extension[1]. I don't use either so haven't been able to test directly yet. But I did use a CLI tool based on the same KDE code[2] and while it doesn't output iCal it does seem to correctly extract the details from a reservation email (I tested on Ryanair).
0: https://apps.kde.org/itinerary/
1: https://www.volkerkrause.eu/2020/01/18/kde-itinerary-nextclo...
2: https://www.volkerkrause.eu/2019/09/21/kde-itinerary-cli-ext...