I've just stumbled upon the std::alignment_of type trait, and its soon-to-be friend std::alignment_of_v. They seem to have been specifically designed to be equivalent to a plain call to alignof, and the future addition of the _v helper indicates that it's not just a legacy bit.
What is the use of std::alignment_of (_v), when we have alignof ?
They are almost completely redundant. As @Revolver noted, they were introduced in different papers, and alignment_of comes from boost nearly verbatim.
But that does not mean the trait is useless.
A template<class...>class can be passed to other templates and used with metaprogramming. Operators like alignof cannot: you would have to write the template<class>class alignment_of before you could pass it to metaprogramming facilities.
Now the same could be said of sizeof needing a std::size_of<class> template.
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The addition of _v was because they swept every ::value integral_constant-type template in std and added a _v variable template. Considering which ones where worthy and which not would be bikeshed painting and nearly pointless: it is easier to do every one than spend effort picking the worthy ones to do. It being done is not evidence the feature is not obsolete.