Can union be templated?

In place of a union you can also use std::variant as of c++17 https://en.cppreference.com/w/cpp/utility/variant

Yes, a particularly useful application is to represent a type simultaneously as a byte array:

template <typename T>
union test
{
    unsigned char ch[sizeof(T)];
    T variable;
};

Yes, it seems that this has always been allowed. A union is a class, and a template is either a function or a class template.

Relevant parts of the standards:

• [temp]

  The declaration in a template-declaration shall

  — declare or define a function or a class, [...]

• [class]

  A union is a class defined with the class-key union

(So one might argue that the new type trait std::is_class is a slight misnomer; the traits are supposed to partition the space of types, and so is_union is a separate, mutually exclusive trait.)