Long story short: there was a hardcoded limit in 1000 values per connection and it was an issue for middle/big size zabbix proxies and/or slow networks.
The limit has been increased to 10 000 (together with other significant improvements) in ZBXNEXT-1804, so the issue is mostly resolved.

But the ZBXNEXT-1804 also asked to make the configurable for zabbix users, but it was not included to the development.
It still might be useful in some cases, for example:

• veeeery slow network,
• very small zabbix proxy on an embedded device,
• ability to control maximal backlog synchronization speed (thinking about ZBX-7702),
• etc.