Can I safely conclude that the sticky bit is not used in current file sysems and reuse the bit for my own purpose.
Even if it's currently unused for non-directories, others have undoubtedly had the same idea and are probably currently using the sticky bit for their own nefarious purposes. The sticky bit is not meant for arbitrary user-defined marks on a file. Use extended attributes instead!
No, you cannot assume that. It's not true for directories. You can make the narrower assumption that it's true for non-directory files.