How behind Ubuntu is Debian, and where can I read this information myself (i.e. is there an official list by either the Debian or the Ubuntu people that says "here's all the stuff that the latest stable Debian is missing from the latest stable Ubuntu"?)
It is not behind all the time: it just follows a different (and more flexible) schedule. [1]
For instance, the current debian stable release entered freeze on 2019-01-12 and was finally released on 2019-07-06. [2]. This means that most software in stable is currently at the latest versions as of the end of 2018. Hence, up until the release announced today, debian stable had more recent (by around 1 year) software than Ubuntu LTS (18.04).
Now and until the next debian stable release (which should happen around summer next year) Ubuntu LTS (20.04) will have newer software than debian stable.
There are a few software packages that do have newer versions in Ubuntu because they are developed by canonical themselves. I can only remember LXC/LXD as examples of these.
I personally run Debian on my personal Desktop, and have no complaints about it at all.
How behind Ubuntu is Debian, and where can I read this information myself (i.e. is there an official list by either the Debian or the Ubuntu people that says "here's all the stuff that the latest stable Debian is missing from the latest stable Ubuntu"?)