Steps to reproduce:

1. Create a trigger
2. Assign a name
3. Directly enter a trigger expression that uses "item_count (item filter)" function

Result:
Observe an error message :

Invalid parameter "/1/expression": incorrect expression starting from "item_count(...

Expected:
A successful creation, as according to our documentation

Except where stated otherwise, all functions listed here are supported in:

Trigger expressions

• Calculated items

https://www.zabbix.com/documentation/6.0/en/manual/appendix/functions/aggregate

and it is not "stated otherwise" for the "item_count (".

Thus we need to state this for the function.

Please, also check if any other functions should be marked as unavailable to be used in trigger expressions.

P.S. If the graphical dialogues addition of the trigger expression is used - i.e. "Add" button is pressed and a function is being selected from the available in combobox - "item_count()" function is not listed (implies that it is unavailable). But still many Zabbix users type in functions directly.