Should a node no longer be in consensus - i.e. their software is out of date - are they a threat to the larger network?
In one case: Should they fork from the, hopefully, larger network and would no longer be able to create valid blocks. The other nodes would reject their blocks as being invalid.
Transaction in these invalid blocks would still be on the wire, i.e. not lost or dropped, so there wouldn’t be fear of loss of these.
There would be a hit on block creation times though. But these nodes wouldn’t be otherwise spamming the network (other than perhaps passing their invalid block during “their” time in the queue though).
Love to hear from everyone else.