In previous versions, the usage of JSON for text components allowed for the unicode escape syntax \uXXXX to insert unicode characters. SNBT does not support this, resulting in a feature regression.
/tellraw @a "\u0041"
MC-279418 Escape sequences don't work anymore due to the change from json to snbt
MC-279232 SNBT text components prevent mixing text styles in lists
MC-279229 SNBT text components prevent \n and \t from working