The only thing worse that comes to mind than repurposing flags is re-using UUIDs, which I have seen some production DB do (no, not mine, thank you very much!).

Reusing the RAM occupied by flags might be do-able in a clean way using guards like the structured enum in Rust, which permits unions (objects that occupy the same space) that always have the right type (i.e., the compiler has knowledge what is in there at each point in time). This mechanism could in theory be extended beyond type-safety to accommodate other contexts in systems programming use cases where memory usage is extremely important.