An AIX admin claimed it could do a live update of even kernel code without a reboot. The solution for most others is clustering, needed to deal with hardware risk anyway. OpenVMS clusters have reportedly gone 20+ years without system downtime. Individual instances needed reboots so little that admins occasionally forgot how to do that. So, yes, there's precedents in other operating systems according to their users.
Note: And as far as OpenVMS, I believe it because those designers built it like their job depended on it not failing. A cluster of that OS shouldn't experience any significant downtime given about everything I've ever read on it.
Note: And as far as OpenVMS, I believe it because those designers built it like their job depended on it not failing. A cluster of that OS shouldn't experience any significant downtime given about everything I've ever read on it.