It took me a bit to hunt down, but it was part of the go 1.4 release in 2014.
the release notes[0] at the time stated
> The implementation of interface values has been modified. In earlier releases, the interface contained a word that was either a pointer or a one-word scalar value, depending on the type of the concrete object stored. This implementation was problematical for the garbage collector, so as of 1.4 interface values always hold a pointer. In running programs, most interface values were pointers anyway, so the effect is minimal, but programs that store integers (for example) in interfaces will see more allocations.
@rsc wrote in some detail[0] about it the initial layout on his blog.
I couldn't find a detailed explanation about how the optimization interacted w/ the gc, but my understanding is that it couldn't discern between pointer and integer values
I can't imagine it was impossible to keep the inline-value optimization; after all, the full type information is always right there in the other word of the interface value, but I think it would have been too complicated/expensive for too little benefit.
The bigger issue IMO is and has been that fat pointers (strings, slices) can't be inlined in interface values. There's definitely some benefit to inlining integers and floats, but the indirection that comes from not inlining them isn't that significant compared to the double-indirection that comes from not inlining strings and slices. There was some discussion on the mailing list IIRC about expanding the interface to 3 words wide (to hold strings at least) or 4 words wide (to hold slices too), but this was rejected as going too far the other way (at the time).
Interestingly, the log/slog package does inline string values at least [1], and demonstrates how such a thing can be done when needed, albeit with a fair deal of complexity.
It was a memory model / two word atomicity problem. The mutator uses two writes, one for type and one for value to create the interface. The GC concurrently reads the 2 words of the interface to see if the value is a pointer or not. This is a race that was considered too expensive / complicated to fix.
