I also just tried this as well, sending an email from a Migadu-based account to one at both Gmail and MXRoute using Mail.app under macOS 15.7.7. Neither included any private IP address info I could find in either headers or raw source. That would be a good leak to know about and as sibling comment said saagarjha definitely knows their stuff, so any tips to replicate would be appreciated.

Is this from iOS? I just tested from MacOS, and the only IPs were for the transit and auth servers.

I'm not actually doubting it. saagarjha knows his stuff. I just don't see it, so maybe I'm holding it wrong.