Last time I checked the practice of using stickers - which can and have been used for a long while now to declare preferred pronouns in our Movement events - isn't documented (here or elsewhere, for the matter).
Topic on Talk:Hackathons/Handbook
Feel free to add it somewhere to the documentation if you like!
For our events specifically we have fill in the blank stickers and/or badges where people are welcome to add anything they would like people to know about them including pronouns, languages, hobbies, or really anything.