I would consider merging (heh) a few of the sentences for clarity:

> The Boolean `mergable` attribute indicates whether the pull request can be automatically merged, while the `merge_commit_sha` attribute holds the SHA of the _test_ merge commit. However,  however, the `merge_commit_sha` attribute is [deprecated](/#expected-changes) and is scheduled for removal in the next version of the API.

Alternative:

> (This _test_ commit is not added to the base branch or the head branch.) The `merge_commit_sha` attribute holds the SHA of the _test_ merge commit; however, this attribute is [deprecated](/#expected-changes) and is scheduled for removal in the next version of the API. The Boolean `mergable` attribute will remain to indicate whether the pull request can be automatically merged.