Considered page https://www.zabbix.com/documentation/4.4/manual/appendix/items/kerberos

As for Step 3
 man kinit:

DESCRIPTION
      kinit obtains and caches an initial ticket-granting ticket for principal.  If principal is absent, kinit chooses an appropriate principal name based on  existing credential cache contents or the local username of the user invoking kinit.  Some options modify the choice of principal name.

so, "zabbix" user most for the command looks like a remote user, i.e. domain user name, not local system name.
This should be clarified. And if so, I'd recommend to use different name, like "[email protected]", to look different from "zabbix" account for daemon on Linux,

As for steps 3 and 4:
it's not clear what shell user should be used to issue commands. Or any user?

Also, I'd add examples when Step 3 is successful and how to check that keytab file has been created.

Different articles how to configure Squid with Kerberos auth may help. For example
https://wiki.squid-cache.org/ConfigExamples/Authenticate/Kerberos

As for ticket expiration is 10h:
Maybe the article should be expanded/changed and zabbix server should be joined to domain, to keep the ticked valid. Not sure here.