I doubt it’s a target unless someone was actively profiling the company in question, building out a plan, and seeking to actively exploit the company’s network from that machine. Even then, how many attackers know IRIX well enough to pull it off?

If we were talking about a networked DOS machine, Windows XP, or even classic MacOS this lack of updates would be more serious… but niche UNIX workstations? Not as certain they’re still targets.