Pretty uncharacteristic for Valve, wonder if something changed in their legal department. In any case, what a shame. This was quite an impressive project.
going by the other comments on the link & elsewhere, seems to be mostly that they don't want to even remotely be in the same context as unlicensed/proprietary nintendo libraries the portal64 dev is using. (also mentioned in the the dev's own message on the screenshot)
Damn. I was just in another thread defending them for taking down that tf2 thing. But Portal64 was a very different kind of thing IMO and seemed more like a demonstration of technological capabilities rather than trying to mow Valve's lawn. I suppose it did seem to use a lot of their assets though.
I have to wonder if this is the work of an actual Valve representative, or if this is a fraudulent takedown notice created by a troll. Given Valve's track record, it seems vastly more likely that this is not a legitimate takedown notice.