That is right, for a long while PowerApps didn’t support complex fields in SharePoint Forms, but it does now, and you might have missed that essential piece of information. Managed Metadata fields are maybe the most versatile and widely used complex SharePoint field type. They use what is called hierarchical set of terms, allowing you to create what ever taxonomical classification needed to describe your data. Term store management includes features like additional labels or synonyms for different terms, support for different languages, as well as moving, copying and deprecating terms.

Currently PowerApps forms supports both single and multi-choice Managed Metadata fields. It supports regular Managed Metadata fields as well as the Enterprise Keywords. The only thing I find missing at this point is support for so called open submission policy of a term set, where users can add their own values straight when filling metadata.