One thing I've been meaning to try, but never got round to, is to stick a URL on an envelope, pointing at a page with an address, and see if the mail (royal mail, in my case) actually deliver it. I suspect they would but that it would take a few extra days. It's no worse than some of the addresses that they do deliver.
It does if it leads to a web page with an address.
What happens when all project maintainers die and the source code disappears?
Hopefully it will never disappear, since Software Heritage and ArchiveTeam will have saved it.
https://www.softwareheritage.org/ https://wiki.archiveteam.org/index.php/Codearchiver
Hope is not a strategy. As much as I hate crypto, something on the blockchain might be more durable. You want something that isn't reliant on any one person or company to continue to exist (though maybe the long now foundation will) and even if Bitcoin goes to zero, I think there will be some die hard true believers to keep running miners even past the built in 2140 expiration date.
What about encoding the address as a QR code?
This should not require any Internet access to view by whoever is scanning it to be sorted for delivery.