Also the l2tpethx interface is also gone.
However, when I attempt to recreate the tunnel, using the exact same command as I initially did, this is the result:
# ip l2tp add tunnel tunnel_id 90 peer_tunnel_id 1 udp_sport 5016 udp_dport 5017 encap udp local 172.16.0.1 remote 172.16.0.10
RTNETLINK answers: Operation not permitted
Changing the tunnel id will not have any effect. However, if I change the port numbers or the remote IP address, I am able to create the tunnel.
What is lingering after I delete a tunnel that is preventing me from creating it again? How can I get rid of whatever is lingering?
Or even better, why does the tunnel stop passing traffic in the first place?
This isn't an OpenL2TP problem - you are using static L2TPv3 tunnels.
I suggest use the kernel's l2tp debugfs to see what state the kernel has about the tunnel.