Display name
This is the profile name friends see. Unicode usually stays. If Discord rejects the save, the string is too long or contains blocked marks — shorten or drop zalgo. The login username stays separate. Nitro vanity and display names are still Unicode-capable on current Discord; Nitro is not required to paste sans bold. Do not pay just to test this Discord fonts page. If you change display name often, friends’ search history still keys off the username. Tell them the typeable name once.
Server nickname
Per-server nicks can be Unicode unless a bot enforces ASCII. If your nick resets on join, that is a bot, not a failed copy from this Discord fonts page. If only one server strips your nick, clone the nick into a DM with yourself to prove the clipboard is fine, then talk to that server’s mods.
Messages
You can paste Unicode into chat. For simple bold/italic, Markdown is cleaner. Use this generator for Fraktur or circled words Markdown cannot make. Announcement channels that need @everyone to work should keep the readable part of the message in ASCII. Fancy the stamp, not the instruction.
Server names and channel topics
Short Unicode stamps work. Heavy glitch stacks can break mobile layout. Keep channel names people must type mostly ASCII. Channel names people type with a hash should stay mostly ASCII. Unicode channel names are hard to jump to from memory.