This is an interesting question. Doesn't Signal rather famously not track anything about who you're talking to? Reputational damage to Signal's brand aside, I expect it'd be hard to sell advertisers on a black hole.
You can have advertisements without tracking. See the original TV ads.
The "tracking" was done by monitoring spikes in sales (using a promotional code) and "focus group" interviews from customers.
These days the accuracy of tracking has improved immensely.
If they did, you wouldn't be able to use a fork or another client because it isn't federated unfortunately.
that doesn't mean you can't fork the client. See telegram for an example of third party apps without federation