`export template` was included in the C++98 standard without any experience, experimental or otherwise. The first implementation was achieved by EDG after enormous pain in the early 2000s, and their advice for any others attempting it was "don't". I'm not aware of anything else quite that egregious, though.

https://www.open-std.org/Jtc1/sc22/wg21/docs/papers/2003/n14...