I am interested in filtering out network interfaces that aren't connected to make things a bit simpler. I tried the filter
{#SNMPVALUE} : ^u|^1
on this page:https://www.zabbix.com/forum/showthread.php?t=37656
I added it to the filter on "Template OS Linux: Network interface discovery" but that just hides all network interfaces. I am not sure if there have been some updates in how I should filter that out in the newer versions of Zabbix.
The forum thread you found talks about discovering SNMP interfaces, while the template you mentioned is for Linux.
For the agent interface discovery, {#SNMPVALUE} is not populated at all, thus your filter never matches.
To filter out interfaces, you would have to figure out the criteria. The built-in interface discovery only returns the interface name, thus there is no way to filter interfaces by another criteria - unless you implement a custom LLD.
Built-in support for filtering agent interfaces by state is requested in the comments of ZBXNEXT-2286.