OpenRC was a C rewrite of what was previously a bunch of bash scripts, but they still implemented the same rich dependency and function-based init logic; it was miles ahead of Debian's init.d scripts before it became OpenRC. OpenRC largely made it faster, and over the years the configuration became (optionally) more declarative and gained features. The actual OpenRC transition was largely seamless for users, because the init script syntax didn't change.
So the year you're looking for isn't when OpenRC was introduced, but rather when Gentoo started using the OpenRC style of init script, which happened much earlier - it's definitely been like that since I started using Gentoo, at least since 2003 or so.
It's actually a misconception that sysvinit is the whole init system; sysvinit is PID 1. Gentoo still uses sysvinit for PID 1 by default. What matters is what your service manager is - that's OpenRC here.