Linux as a network printer device (Raw, port 9100)

I understand that network printers use a protocol called RAW (or Jetdirect), is that correct?

Yes. There are two network protocols for print jobs that are still relevant today: LPD, running on TCP port 515, and JetDirect, running on TCP port 9100. LPD was designed by Berkeley for BSD UNIX in the old days. It's pretty much obsolete now, but it's still available on many printers, presumably for compatibility with old servers. JetDirect was designed by HP in the early 1990's, and it's simpler and somewhat faster than LPD. You should probably use JetDirect whenever available.

I have a RS/6000 with AIX 5 that finds and works with any kind of 'native' network printer. And I would like to expose in the network a USB printer like a network printer (autonomous device), using Linux (preferably Debian) to do that.

Install CUPS on a Linux machine, and connect the printer to it. You don't even need a full machine for that, a Raspberry Pi or similar, or a home router with an USB port and running OpenWRT or DD-WRT, would work just fine.

CUPS allows you to share printer with (at least) IPP, LPD and samba protocols out of the box. IPP is supported by most operation systems and IBM was part of the Printing Working Group which came up with IPP so it's likely supported in AIX as well.

Also, samba has AIX protocol so one or the other should work. search for AIX

But none of the documents I came across referenced AIX version so I can't be sure if AIX 5 is supported or not.

The way it works anyhow is that you plug the printer to server with CUPS installed and select the method of sharing, broadcast settings etc. It's straightforward. Then clients can connect to it and it'll show up as network printer.