Haven't "launchers" existed for decades at this point though? I remember Crunchbang (RIP) having something similar for example, and that must have been almost two decades ago at this point.

There are a lot of them yeah.

In this case, Walker is the launcher being used in Omarchy, https://github.com/abenz1267/walker. It's not specific to Omarchy.

One of Walker's features is being able to create your own custom menus quite easily with shell scripts.