Linux
The IP implementation in Linux defaults to the weak host model.
FreeBSD
The IP implementation in FreeBSD partially implement the Strong End System (i.e., host) model, but it’s disabled by default and only works in non-forwarding mode. Since FreeBSD 14, you need to set net.inet.rfc1122_strong_es to enable this feature. See inet(4) for details.
OpenBSD
Since OpenBSD 6.6, the strong host model is the default in non-forwarding mode.
PF (Packet Filter) offers a Unicast Reverse Path Forwarding (uRPF) feature. It has the same functionality as antispoof, and can be used to implement the strong host model with IP forwarding enabled.
The uRPF check can be performed on packets by using the urpf-failed keyword in filter rules:
block in quick from urpf-failed label uRPF
References
https://blog.kanbach.org/post/network-security-implications-of-host-models/