I used the kernel AX.25 stack on and off so I'm sad to see it go. I'm in the minority, I guess, because I don't use anything AGWPE and instead exclusively use UDP over IP with vanilla sockets. IP-over-packet just feels more fun than APRS.

I spent the last 24 hours hacking together a simple KISS<-->TUN adapter that connects Direwolf's KISS-over-TCP server to a TUN
interface. It seems to be working as a drop-in replacement. It did require implementing ARP in userspace (IPv4 only right now). But moving the ARP implementation to a dedicated AX.25 implementation means it can (eventually) be tuned to be more suitable for packet radio use (the kernel ARP resolver by default is a bit too aggressive for 1200 baud).

While it's still sad to lose it, it doesn't seem like it's irreplaceable. It's still possible to pipe packets between Direwolf and the kernel IP stack.