> Would it be acceptable to have a newer snapd for stretch in something like -backports?
I don't think so. I think it's a reasonable expectation that the certbot snap, with its plugins, works out of the box with the snapd that shipped with Debian. Otherwise, you're effectively saying that you don't care about regressing snapd as shipped on non-Ubuntu distributions, and that snaps do not operate universally across distributions as advertised.
This also appears to be a recent regression. Apparently something changed in the core snap that caused the regression.
Note that the certbot snap is based on core20, so I'm puzzled as to why installing "core" triggers the issue.
> Would it be acceptable to have a newer snapd for stretch in something like -backports?
I don't think so. I think it's a reasonable expectation that the certbot snap, with its plugins, works out of the box with the snapd that shipped with Debian. Otherwise, you're effectively saying that you don't care about regressing snapd as shipped on non-Ubuntu distributions, and that snaps do not operate universally across distributions as advertised.
This also appears to be a recent regression. Apparently something changed in the core snap that caused the regression.
Note that the certbot snap is based on core20, so I'm puzzled as to why installing "core" triggers the issue.