i thought there was an issue about this already, but i can't find it.

in any case, validation would simply check that at least one item is referenced and not allow saving triggers if not.

it would be similar how one could try to create a trigger that would only include a user macro.

Probably richlv meant ZBXNEXT-1791 which contains related discussion.
I'm not sure who is duplicate

<richlv> no, the discussion i had in mind was a couple years old at least

When considered for implementation (what I appreciate) keep in mind value resolution by macros (e.g. {ITEM.VALUE<1-9>})
There should still be the possibility to resolve the corresponding value in trigger names etc.

Kind of related: ZBXNEXT-2469

ZBXNEXT-3688 suggests using global macros for time(), now() and other such functions

>it would be similar how one could try to create a trigger that would only include a user macro.

Such trigger (based on now() or time(), for example) can exists and can be potentially useful. All problems lying on the database level where each trigger must be connected to some item. 
I think that now() (for example) is _really_ based on not-defined-yet item, but this item is in the zabbix_internal items set: it is something like zabbix[time,unix_ts].

It has been implemented in Zabbix 5.4. Now you can use time and date functions without references to any item, see https://www.zabbix.com/documentation/5.4/manual/appendix/functions/time