No, like on most other archs, people use tagged pointers in lisp and other runtimes on amd64 all the time. But the machine (like most ISAs) doesn't ignore any b

No, like on most other archs, people use tagged pointers in lisp and other runtimes on amd64 all the time. But the machine (like most ISAs) doesn't ignore any bits so you have to clear the tag bits before dereferencing. You may be thinking of the fact that a full 64 bit VA space isn't supported in current implementations. But this has no impact on use of tagging compared to most other archs.