Docker could run a minimal pid1 in each container to address this. Though if this had been a big issue I guess this would have been already fixed.
Naturally, a proof of concept of the problem would be great. (Let's say a Dockerfile.)
It's usually fairly simple to fix (e.g. for Consul above, I raised it with the Consul guys and they said they'd look at adding waiting on children to it as a precaution - it's just a couple of lines -, but people building containers could also introduce a minimal init, or you can write your health checks to guard against it), but it happens all over the place, and people are often unaware and so not on the lookout for it and it may not be immediately obvious.
The reason I raised it as an issue for Consul, for example, even though it wasn't really their fault, but an issue with the containers, is that people need to be aware of the problem when packaging the containers, need to be aware that a given application may spawn children, and that they may not wait for them. Even a lot of people aware of the zombie issue end up packaging software that they didn't realise where spawning child processes that could end up as zombies (in this case, it took running it in a container without a proper pid 1, using health checks which not everyone will do, and writing the health checks in a particular way in order to notice the effects).
Thankfully there are a number of tiny little inits. E.g. there's suckless sinit [1], Tini[2] , and here's a tiny little proof of concept Go init [3] I wrote (though frankly, suckless or Tini compiled with musl will give you a much smaller binary) as what little you actually need to do is very trivial.
[1] http://git.suckless.org/sinit
Also thanks for the Consul example, makes it much-much easier to see the issue and argue for a general solution. (So not every random app/project/service/daemon has to implement pid1 functionality.)
That doesn't fix the issue since you need to know about the issue and accept that it exists, at that point you can just as easily use one of the micro-inits available.
The alternative is to enable it by default, but now you've broken BC for the weirdo who actually expects orphan processes to be adopted by the root process they're starting.
We all pick our battles - including me of course!