Thanks to the people who responded plus a little searching through this bot's posts ( https://mastodon.social/@trendytoots ) I have extended the list. Please feel free to respond with any more you know and can comfortably share, maybe with a description of what it's for, and a suggestion for what to call this kind of practice (or maybe it has an official name?). I'd really love to see examples from a broader variety of languages!
Note: if I'm bothering the people who use these hashtags, please let me know! I can remove the "#" or the whole hashtag from the list