Yes, Unix = Linux for all intents and purposes in 2026. When I started in the early 90s, there was a galaxy of Unices - SunOS/Solaris, HP-UX, AIX, and many others people people don't even remember (ConvexOS anyone?). Heck, back then if you wanted to run a particular software for your industry, you might have to buy and install a specific proprietary Unix.

Today? It's all Linux or Windows (of course, now also Web/SaaS). All these different Unices have fallen by the wayside in favor of Linux. Yes, some are still around but the mindshare has been in Linux for the last 20 years.

I suspect you could rip out a ton of autoconf rules and no one would notice.