Steps to reproduce:

1. Have proxies on 3.4.2 (Higher versions might have this problem too)
2. Upgrade server/frontend to 3.4.10
3. Check Zabbix proxy logs.

Result:
877:20180607:093847.463 failed to update local proxy configuration copy: unexpected field "hosts.available"
877:20180607:094848.228 received configuration data from server at "<host>", datalen 1208671
877:20180607:094848.229 failed to update local proxy configuration copy: unexpected field "hosts.available"
877:20180607:095848.936 received configuration data from server at "<host>", datalen 1208671
877:20180607:095848.938 failed to update local proxy configuration copy: unexpected field "hosts.available"
Expected:
Database updates can apply.

I assume this is a bug, in my Zabbix course I remember being told that Zabbix proxy and server stay compatible. The upgrade to Zabbix server 3.4.10 broke proxy compatibility with proxy 3.4.2. 
(I have a proxy running 3.4.10 that syncs just fine, I have no idea if any versions after 3.4.2 also have this issue). 

As I did not see this in the upgrade notes on the website, and no upgrade nodes seem to be available in the manual : https://www.zabbix.com/documentation/3.4/manual/installation/upgrade_notes_349