I don’t see how one library for one language having some undesirable feature implies “If a language needs to rely on pointer tags for GC, avoid that language.”,

I don’t see how one library for one language having some undesirable feature implies “If a language needs to rely on pointer tags for GC, avoid that language.”, especially not since using tagged pointers does not necessarily imply limiting the range of integers. An implementation can choose to implement ‘small’ objects as tagged pointers, and use a pointer to an ‘integer’ object for objects it cannot ’hide’ inside a pointer.