Are there any new requirements for port forwarding for ASL3? As well, with a modern router, is it possible to have port forwarding active with a dynamic IP address, or is static still necessary?
Just the IAX port.
On the dynamic vs static IP: this is going to depend on your router. Usually a router forwards based on IPs...so you need to make sure the local IP never changes. some routers will do it by device and can track; but I would recommend giving the device a local static IP just to avoid issues in the future.
Only if accepting inbound connections is a requirement.
Not a requirement for client ASL usage.
Allstar's configuration may allow for it; however IAX wasn't really designed to operate that way and the actual protocol expects both sides to be connectable.
"It's how the pros do it"
So, to be technically accurate here.... IAX as used by ASL today requires inbound IAX to any system using it. In many home cases, this "just works" if you're behind a NAT device without port forwarding. This is because outbound IAX UDP/4569 gets NAT'd still to UDP/4569 using the WAN-side public IP address and the state engine on the firewall allows the return traffic. But that's only reliable for "low traffic" use cases where your likelihood of UDP/4569 being available is almost guaranteed. For anything else - CGNAT networks from providers, corporate networks, etc. - you must specifically permit inbound IAX on the same port you configured. Otherwise, you'll end up with a random port outbound and the return traffic has no state-match in the firewall and is dropped. CGNAT when you have a double-hops makes that impossible which is why ASL doesn't work at all on CGNAT systems.
In the (hopefully near) future, the keyed upgrade of the ASL protocol will eliminate this and leverage arbitrary ports on callers to avoid CGNAT issues. It's in design now.