Sadly, both sides of this argument seem to have diverged from the facts of TTN and LoRaWAN.
Actually, every gateway connected to TTN is for public use from the moment of its connection. There are no private or “test” gateways.
The impact of a one-time or otherwise rare configuration downlink is next to nil. Downlink for purpose of configuration is quite normal, appropriate operation - after all, that’s what OTAA (the officially recommended method solution!) fundamentally is as well - a downlink to configure the node with a shorthand identity to use on the network and some other related details of the network.
Now, what is actually detrimental to the network?
One things that’s drastically worse than sending a downlink through a gateway, is unplugging it! Sending a downlink “removes the gateway from the network” for anywhere from say .1 to maybe 1.8 seconds (possibly a bit more if it runs out of airtime). In contrast, unplugging a gateways means it’s gone gone.
And that’s actually a lot worse. LoRaWAN’s ADR scheme adapts pretty quickly to the appearance of a gateway, in that nodes (including other people’s nodes) start transitioning to a faster, shorter range data mode to take advantage of a newly closest gateway.
But when the new gateway vanishes, it can take hours to even days for nodes to roll back to a data rate where they have the range to reach the next nearest surviving gateway.
So in actuality, if you’re going to plug in a gateway, feel free to send the occasional appropriate downlink through it. But if you want to be a good TTN citizen, then don’t plug in a gateway temporarily, but rather put it somewhere that you can leave it running for months on end.
The only private, temporary, test gateways are gateways that never connect to TTN’s severs at all.
(That said, using a 50 ohm terminator on the gateway instead of antenna would probably confine receive coverage to tens of meter’s radius, minimizing impact on others - it’s quite possible traffic to your nodes would go through someone else’s gateway at that point, but presumably the reason for doing this at all would be when you’re too far from any other gateway for intended tests)