The 3D Realms Legacy history page states:
The 3D Realms store still has pages up for Keen, but it's "Unavailable".2020
- Jul 9: Wolfenstein 3D, Keen Vorticons, & Keen Galaxy no longer sold by Apogee/3DR
Joe Siegler wrote a page on his time at Apogee, and said:
So that would appear to be that — the end of an era! I guess Steam is the only place to buy new copies of Keen now…Apogee/3D Realms sold Keen Vorticons in shareware from 14 Dec 1990 until 9 Jul 2020, when the original shareware deal between Apogee & id Software from many a moon ago was finally brought to a close.