I by default get the Apple Wallet boarding pass and never delete any (they move to a dedicated "Expired" section after flying for most airlines - some don't seem to set the expiration date and those I had to sadly delete myself).

So perhaps going through those, if you have them, helps. I had almost all my boarding passes since 2013 in there and therefore Flighty now also has all my flights from the last 10+ years in there.

Older flights aren't always recognized by Flighty properly anyway and lack a lot of information, so it's not super useful to have them in there.