According to the documentation the configuration parameter TLSAccept is mandatory "if TLS certificate or PSK parameters are defined". But in fact the proxy also works without setting this parameter. In contrast the agent fails to start if TLSAccept is missing.

The relevant proxy configuration parameters are set as follows:

ProxyMode=0
TLSConnect=psk
TLSPSKIdentity=proxy_identity
TLSPSKFile=/etc/zabbix/proxy_psk_file

So if it really needs to be set the proxy should behave like the agent and refuse to start. Otherwise the documentation should be corrected.