I think Linux is perfectly UNIX in terms of userland and 'spirit', if you will. It's not UNIX officially, but that's because nobody bothered to pay for the licensing, since it's not worth it, not because it couldn't get certified as UNIX if somebody wanted to do it - in fact D. Ritchie himself said:
"I think the Linux phenomenon is quite delightful, because it draws so strongly on the basis that Unix provided. Linux seems to be among the healthiest of the direct Unix derivatives, though there are also the various BSD systems as well as the more official offerings from the workstation and mainframe manufacturers."
K. Thompson is also using Linux. I think that's the ultimate testament to it being 'Unix', unless you want the trademark for marketing purposes, like Apple does.
As per[1], "According to a 2009 interview, Thompson now uses a Linux-based operating system." He probably uses multiple systems, including macOS in some capacity, am sure, but it looks like he uses/used Linux.
I can't find the source right now but Russ Cox posted to some newsgroup or mailing list that most golang developers use OS X these days (rsc, rob and ken for sure).
"I think the Linux phenomenon is quite delightful, because it draws so strongly on the basis that Unix provided. Linux seems to be among the healthiest of the direct Unix derivatives, though there are also the various BSD systems as well as the more official offerings from the workstation and mainframe manufacturers."
K. Thompson is also using Linux. I think that's the ultimate testament to it being 'Unix', unless you want the trademark for marketing purposes, like Apple does.