C++20: Non-capturing lambda in non-type template parameter

Does C++20 allow a non-capturing lambda decayed to a function pointer to be passed directly as a non-type template parameter?

Yes.

Indeed, you can go one step further - you don't even need to convert the lambda to a function pointer. You can just provide the lambda. This is valid C++20:

using Y = S<[](int x) -> void { std::cout << x << " hello\n"; }>;

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The rule we have in C++20 is that lambdas are now allowed in unevaluated contexts (P0315). Among many other wording changes there, this paper struck the rule that prevented lambdas from being used in template arguments (C++17's [expr.prim.lambda]/2):

A lambda-expression shall not appear in an unevaluated operand, in a template-argument, in an alias-declaration, in a typedef declaration, or in the declaration of a function or function template outside its function body and default arguments.

That clause does not exist anymore in C++20.

Removing this restriction allows the lambda to be used as a template argument, and the conversion from captureless lambda to function pointer was already constexpr in C++17. clang simply does not implement this feature yet (using T = decltype([]{}); compiles on gcc, not yet on clang). I wouldn't call this a clang bug yet, it's just a clang not-yet-implemented-feature (lambdas in unevaluated contexts is not yet listed as implemented in the cppreference compiler support page).

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C++20 non-type template parameters (P1907) allows even dropping the + because captureless lambdas count as structural types ([temp.param]/7) by way of simply not having any data members at all.