I mean, sure, I've done this too with a handful of scripts. But is this something you do via .debs? I'm asking specifically about how to handle this with plain Debian/Ubuntu packaging.

I do it similarly. I run multiple versions of a service at once with systemd service files. It gives me the same stuff as containers - cgroups, isolation, logging, service definitions and automation with ansible, but its easier on my feeble psyche.