The Electrum plugin was discontinued due to low interest.

But it looks like their API is still up. Not sure for how long, though.

There is no public documentation about their endpoints, but you can take a look at their plugin's source code to get an idea: https://github.com/chipmixer/electrum/blob/plugin/plugins/chipmixer/chipmixer.py

E.g: https://chipmixer.com/api/v1/new_session