But are those available for "an old SGI Irix workstation or even a DEC Ultrix machine" ?

Firewall should be a separate machine. I am using 2 x 2012/2013 fanless industrial PCs next to the server rack(one main + 1 spare). The idea is that you're not losing anything or changing anything on the original website/machine. You just add one in front of it and you're done.

Would work with anything ethernet enabled as far as I know. I never worked with SGI Irix or DEC Ultrix but I assume that they can handle ethernet connections if they are serving websites.

Buying a raspberry pi to act as an nginx proxy probably costs less than the yearly electricity cost of running a machine like that.

I believe the suggestion is that you proxy them via another system.

Which will if you listen to the advice of HN will be an ec2 micro instance. Which means you're handing more of the internet over to "the big guys" exactly as the author of the article points out.

I would say depending on the traffic a raspberry pi should suffice?

Good point. That would be more than enough for low traffic.

Of course it's readily available, you have a $5000 Cisco ASA or F5 appliance sitting next to the $50000 SGI or HP workstation, to perform SSL termination and firewall.

The costs are off.

I have an SGI workstation and an HP workstation next to me. They cost $0.

That was >15 years ago, freecycled. Pretty sure they are less than $0 by now!