For what it's worth, compaction with my mark-region GC for SBCL follows "An algorithm for parallel incremental compaction" <https://dl.acm.org/doi/10.1145/773039.512442> which instead logs all references to the objects we want to move while tracing, rather than using forwarding pointers. Well, not quite - the runtime likes to eagerly copy lists to get them contiguous after copying, and so we still use forwarding pointers to avoid copying cons cells twice.

(The mutator also won't ever see forwarding pointers regardless, if you have a STW GC. And if that GC works.)