How can I get access to the source code of the form? Is this feature gone?

It looks like the source code publishing mode is not active anymore.

How can I get access to it?

This is was the only way forms looked good using them on a custom domain.

Lokking forward to hearing from you soon!!

The form from your screenshot is a Card Form. Please note that the Source Code embed option is only available with the Classic Forms. 

Cards or Classic: Which One Should I Use?