Some companies are still offering OpenVZ because it allows to heavily oversell RAM, CPU and storage bandwidth with somewhat graceful degradation of guest performance.
I think OpenVZ has kernel patches for RHEL7's 3.10.x kernel, which is supported by Red Hat till 2024. So in theory it's possible to have a secure OpenVZ VM but in reality bad practices (like heavy overselling the hardware) flock together.
I'd have thought even LXC would have universally taken over by now. Who is still running CentOS 6 as a host?