I have only tried square brackets, as they are used in Host Group names

Steps to reproduce:

1. Create Host Group named with "[]", i.e. [ABC]/Test
2. Create Aggregated item with this host group

Result:

grpsum["\[ABC\]/Test","example.key","last"]

Status: Not supported
Info: None of the groups in list is correct.

grpsum["[ABC]/Test","example.key","last"]

Status: Not supported
Info: None of the groups in list is correct.**

grpsum["ABC/test","example.key","last"]

Status: Enabled

Expected:
based on documentation, expected behavior is any unicode character in a quoted string
https://www.zabbix.com/documentation/current/manual/config/items/item/key