Thanks! I meant to remove this immediately after r38 was released, but there was a lot of other changes piled up around that time due to the change freeze which I had to merge, and then I just simply forgot about this.

Regarding the changelog, I think it you can make an argument either way. The term 'deprecate' generally means a feature is discouraged, but it is still available for use. In this case the behavior of the sixel option has been changed so that it is a no-op (but raises a helpful warning for users), instead of performing its intended original functionality.

So I'm fine with just leaving the changelog as is - in any case even after this PR is merged, attempting to use set sixel true will still result in some kind of error message, so users will notice that there is something wrong with their configuration.