Task lists (or checkboxes, tasklists) work with most characters enclosed with [ ], not just x os space. Even emojis seem to work, with the exception of 🔁, for example. Here’s a snippet I tried in Obsidian.
- [ ]
- [x]
- [X]
- [a]
- [5]
- [?]
- [✔]
- [🔁]
- [❌]
Obsidian mostly supports Github Flavored Markdown, and the GFM spec supports any enclosed character.
I’m on Obsidian v0.12.15, Windows 10.