The zone files do not contain domain names that have no name servers. So that means that if such a domain was to expire and be deleted, this would not show up in the zone file. But there are not so many domain names with no name servers. In practice, if you compare two zone files on a daily basis (ie current release vs yesterday), you can see which domains were added (new regs generally), and which ones were removed (usually because they entered redemption status).
For the gTLDs, the registrar don't need the zone files to compute lists of deleting domains - they receive lists from the registries.
The zone file is just a snapshot of the domain names provisioned in the DNS.
This is not the registry database. The registry has its own backend that is a fully fledged database. The zone files do not contain whois info etc. Just the domain names, name servers and other DNS records like glue records or DNSSEC data.