irc :

Q : Would it work if i only update the server to 3.4 and keep proxies to 3.2 ?

A : There's one recent exception where 3.4 server will support data collection from 3.2 proxies

Q : is the compatibility documented ?
https://www.zabbix.com/documentation/3.4/manual/appendix/compatibility
Both Zabbix 3.4 proxies and Zabbix 3.4 server are supported to work only with Zabbix 3.4 server and Zabbix 3.4 proxies respectively.

A: Interesting. it was publicly mentioned somewhere, maybe at some zabconf2017 talk , but indeed seems not to be documented properly
Feel free to create a documentation bugreport