I wrote little paper on this technique in school and did some practical tests, at the time I was actually unable to find an example of consumer grade router that it didn't work on! But my resources were rather limited, they certainly do exist.
Even in the presence of a conntrack entry created by an earlier outbound SYN,!ACK ?
Got a source?
I don't have a convenient source though.
> Plenty of setups block incoming SYN,!ACK packets
Nowhere close to being "plenty". It's doable, but this is extremely niche.
I can't really imagine why you would do it for NAT'd v4 since you can't avoid the connection tracking overhead, but you certainly could, and I don't doubt OP has run into it in the wild. I've seen much weirder firewall rules :)
What are the obvious reasons? If you're protecting a client system, you don't want to allow in any bare SYNs. (And for that matter, if you're protecting a server, you probably want to discard ill-targeted bare SYNs without consulting conntrack anyway, just as a matter of avoiding extra CPU work.)
All inbound packets are matched against existing sessions. In this case none will turn up, so the packet will go through the "new session" flow and be subject to the same filtering as a bare SYN. Look up how connection tracking works, e.g. in the Linux kernel, it's rather simple and logical.