Context: we often have users who use Latin-lookalikes in their name—e.g., Cyrillic/Greek or maths symbols—emoji, etc. These seem like they'd be barriers.
Our nickname policy is broadly accessibility to all members:
1) Obviously, it has to conform with other rules about not being hateful, etc.
2) Can be found via @mention menu with any English keyboard layout;
2) Legible and identifiable as a personal identity to English speakers (so no 漢字 hànzì-only nicknames for example);
etc.