How do i send an ICMP Dest port unreachable on an irule?

It appears (on 12.1.1, at least) that the behavior of the

reject

command differs based on whether address translation is enabled. When it is, as I say, an ICMP Port Unreachable message is returned (for UDP traffic). When it is disabled, the behavior you see occurs.

There is no way to send a specific, explicit ICMP response from an iRule. However, a "Reject" type server will send an ICMP Port Unreachable in any case. So, you could create a "Reject" virtual server that is bound to no VLAN:

ltm virtual vs-reject {
    destination 0.0.0.0:any
    mask any
    profiles {
        fastL4 { }
    }
    reject
    source 0.0.0.0/0
    translate-address enabled
    translate-port enabled
    vlans-enabled
    vs-index 6
}

Then, in your iRule, instead of using

reject

, forward rejects to this VS:

when CLIENT_ACCEPTED {
   if { ![class match [IP::client_addr] equals server_pools] }{
       virtual vs-reject
   }
}

(Notice that the explicit

forward

branch is unnecessary because the VS type is already Forwarding). For me, this produces an identical result for classic

traceroute

(using UDP segments bound for random high-numbered ports), which you appear to be testing here.