Zapchat, by TAG.
Zapchat is built on Signal-iOS, which is published by Signal Messenger under the GNU Affero General Public License, version 3. Zapchat is a modified version of it, so Zapchat is covered by the same licence.
Signal's own source is public at the link above. What that copy does not include are the changes that make Zapchat a WhatsApp client rather than a Signal one, and those are what the offer below is for.
Write to support@tag.org and ask for it. Say which version you have — Zapchat shows it at the bottom of Settings, as "Zapchat 1.0 (13)" or similar — and we will send you the complete corresponding source for that build.
It is free. You do not have to explain why you want it, and you do not have to be a customer: this offer is open to anyone who has a copy of the app.
This offer stands for at least three years from the date we last distributed that version.
The full source of the app as it was built, including our modifications, and the scripts needed to build it.
It does not include our signing certificates, push keys or server credentials. Those are not part of the program's source — they are the keys to our own systems, and handing them out would give anyone access to our customers' accounts. Everything needed to build and run your own copy is there.
Zapchat talks to WhatsApp through a small server we wrote ourselves. That server shares no code with Signal, so it is not covered by Signal's licence and is not included in the offer above. It is described in our privacy policy, which explains what it holds and for how long.
Questions about any of this go to support@tag.org.