This is going to be a problem, considering (more and more unfortunate) Ubuntu's popularity. Scripts will continue to be written against Ubuntu, and if they do not work on your Debian or whatever, it's your problem.
Same thing that happens with Alpine's shell, or macOS, or the BSDs — I work with shell all the time and often run into scripts that should work on non-bash shells, with non-GNU coreutils, but don't, because nobody cared to test them anywhere besides Ubuntu, which until now at least had the same environment as most other Linux distributions.
More pain incoming, for no technical reason at all. Canonical used to feel like a force for the good, but doesn't anymore.
I try to execute my scripts with dash, but I am not sure if that is enough. Do you have a good way recommendation to check, that you're not relying on non-portable behaviour short of installing another distro, that doesn't take much effort.
Working on macOS for years taught me that most people are going to write code that supports what they can test even if there's a better way that works across more systems. Heck, even using 'sed -i' breaks if you go from macOS to Linux or vice-versa but if you don't have a Mac you wouldn't know.
Meanwhile, this is a rewrite of `date` (and other coreutils) with the goal of being perfectly compatible with GNU coreutils (even using the coreutils test cases), which means that differences between the two are going to reduce, not expand.
What you're complaining about here is "people only test on one platform" and your solution is that everything should stay the same and never change, and we should have homogeneity across all platforms forever so that you don't have to deal with a shell script that doesn't work. The actual solution is for more people to become aware of these differences and why best practices exist.
Note that recently Ubuntu and Debian switched /bin/sh to dash instead of bash, which then resulted in a lot of people having to fix their /bin/sh scripts to remove bashisms which then improves things for everyone across all platforms. Now Ubuntu switches to uutils and we find they have a bug in `date` because GNU coreutils didn't have a test for that either; now coreutils has a test for it too so that they don't have a regression in the future, and everyone's software gets better.
Same thing that happens with Alpine's shell, or macOS, or the BSDs — I work with shell all the time and often run into scripts that should work on non-bash shells, with non-GNU coreutils, but don't, because nobody cared to test them anywhere besides Ubuntu, which until now at least had the same environment as most other Linux distributions.
More pain incoming, for no technical reason at all. Canonical used to feel like a force for the good, but doesn't anymore.