читаю, не вижу нигде что это будет билтином и новыми кивордами
The match and case keywords are soft keywords, i.e. they are not reserved words in other grammatical contexts (including at the start of a line if there is no colon where expected). This implies that they are recognized as keywords when part of a match statement or case block only, and are allowed to be used in all other contexts as variable or argument names.