https://gerrit.wikimedia.org/r/#/c/199579/ (Use site.lang instead of site.code) was a breaking change, not backwards compatible.

It allowed scripts to fallback to English undesirably, such as when their code was wikidata or commons. e.g. T94680

It also caused site specific logic to not be used. e.g. category.py has a special configuration for test.wikipedia, which now isnt used. (see cfd_templates)
More examples can be found in git grep "'commons':"