Does completing a course grant a Mastery badge for the course?

No- that is not a badge. If you are thinking that the badge is a challenge patch, no, either- challenge patches are supposed to be for a specific subject in a course, like "2nd grade: Place Value". "2nd grade" is the course, and "Place Value" is the subject.

Thank you. I noticed some people have badges like 'Master of Arithmetic'. Do you know how to get such a badge?

Those are old badges that are already "retired" and they cannot be given out any more.

But, they may (or may not) return sometime in the future as we move from our current code base.

Yes, that is true.

For example, 1000 Kelvin will be back as soon as KA finishes migrating to the Go codebase.

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