Usage limits
None of the APIs currently has a pricing model or anything similar. They’re all completely free to use.
Request limits
Every endpoint is limited differently, due to the individual computational work that’s done by the backend. The limit corresponding to the endpoint will be visible on its documentation page.
Attribution
Attribution in general is not required, but it would be a really handsome move by anyone using my APIs.
A simple link to https://witzdam.com somewhere at the bottom of the would totally be enough.