From what I have known and used, PuTTY seems to be the oldest software still in beta (0.79 at the moment), with it's first version released sometime in 1998, not sure about the exact date.
PuTTY's first Git commit shows "Fri Jan 8 13:02:13 1999" for beta 0.43, from r11 of SVN.
https://www.chiark.greenend.org.uk/~sgtatham/putty/latest.html