Commit 8649ff6d removed various metadata fields, including pixiv_ugoira_frame_data.
Apparently it is possible to use the only query parameter to restore certain fields if necessary, but there doesn't seem a way to do that for pixiv_ugoira_frame_data. I've tried various ways, which all failed, and the available_includes in this commit also don't mention anything about it.
For example https://danbooru.donmai.us/posts/3788150.json?only=pixiv_ugoira_frame_data,pixiv_id only lists the Pixiv ID, even though it's an ugoira.
Updated