I had a script using the Python module zbxsend, which implements the Zabbix sender protocol. Apparently this implementation uses timestamps with a sub-second precision in the JSON. I'm not sure whether this section was interpreted as expected, but it generally worked.

It was only a while after my upgrade to 3.0 that I noticed, that this no longer works: The item data is refused. It used to work in 2.2.7. This breaking change should be documented. While zabbix_sender will warn you, custom implementations may fail silently. Nevertheless existing scripts will fail.

I also wonder if Zabbix could interpret the sub-second section properly, since the ns field is in place anyway.