[rollcat]

Well "modern" and "longevity" are kinda at odds with each other, so it may depend on where you choose your arbitrary cut-off point, but I'd say OpenBSD could be one such example. They do sometimes drop support for architectures (RIP VAX), but that usually happens when the last such box in question physically dies. Otherwise they've maintained their bi-yearly release cycle and kept the system small, clean, robust, and secure for what's now decades.

http://undeadly.org/cgi?action=article&sid=20160309192510

Also, they are the upstream project for OpenSSH, which is among the most ubiquitous pieces of software on the planet. (Consider supporting them: https://openbsd.org/donations.html)