Technology

Calling Docker “modern” is a stretch, as it’s not much more than glorified Solaris Zone

And yet the OS you're using doesn't support it. Hmm.

OpenBSD does not have jails.

Sorry, I didn't realise that these are FreeBSD-specific (I guess? I'm not too familiar with BSD)

Which feature of a federated web application requires modern tooling?

Deployment. All web apps and APIs are moving towards containerization - Docker for smaller scale deployments, and Kubernetes for large-scale deployments.

except with better isolation How so?

I didn't think jails had CPU, memory, or process limits similar to what cgroup2 provides, but it looks like this functionality was added to FreeBSD at some point. Sorry for the incorrect information.

Actually, not using Docker prevents a number of security holes/issues.

Sure, Docker has had a few issues, but overall it's more secure to run your apps in Docker containers. If an app gets compromised, the attacker will generally be stuck inside the Docker container rather than getting access to your entire system. If you're worried about (very rare) container escape security holes, using unprivileged containers helps - You can run the app inside the container as an unprivileged user, and you can also run the entire Docker container as an unprivileged user on the host system.

And security patches impossible.

Security patches are easier than if you used a tar archive to install the software. With a tar archive you threw into /opt or whatever, the app and its config/data are often stored together, so you need to be mindful of things like not overriding customized config files. Since Dockers containers are immutable and all the actual data is stored elsewhere, it's always safe to delete the container and replace it with a patched version.