I'm looking at data.json from Itch.getGameData(). This is a very helpful api function, but the tags array seems to always be empty. How can I get a list of tags for a given game?

This is speculation.

That api is not meant to use for other people. It is meant to use for the publisher to include their own game on their own page. Not for someone to replicate the itch catalogue.

That there is a tag entry could be an artefact of dumping the necessary data, since only the data to display for selling the game is needed. During implementing it once was considered to transfer the tags, but was later discarded, since the marketing of the game has to be done at a different place than itch, and tags are only relevant while being on itch. But they forgot to remove the initialisation of the array, hence it still appears as an empty thing.