According to the net-snmp source and RFC1157, SNMP community field has type OCTET STRING / char, so it may contain any of char symbols.
In the current CLI, a community name can use only the limited list of characters: https://github.com/vyos/vyos-1x/blob/846e306700af191d22dc874992bbf5f04d2799c4/interface-definitions/snmp.xml.in#L16.
We should review the possibility to extend it to the full set of supported characters