When it comes to learning how to program there are a few things you can do:

1. Read a textbook
2. Watch videos
3. Read some source code

Of these options I find the best way to truly learn how to program is to read someone else's program and try and understand it. For example recently I've been working on my own dmenu clone for Wayland. Throughout working on it instead of looking for tutorials on how to render a square using pixman I decided to take a look at dtao which is a clone of dzen for Wayland. By just reading the code and messing around with the program I was able to get an understanding for how rendering is done in pixman.

Now you may be asking yourself something like: "But what does this have to do with suckless software?". The answer to that is in their philosophy which is about: "keeping things simple, minimal and usable". The idea of keeping things minimal and useable allows them to create wonderful programs that not only work, but also showcase how to do things without extra fluff that something like i3 might have.

Even if you don't like suckless software it still serves as a great place to learn how to do the bare minimum. And for those who do enjoy using it, it can serve as a great starting place to hack upon until you get the software of your dreams.