Here's some source code that it's known to be preserved, but has not been publicly released for one reason or another:
NFL Blitz (1997), 720° (1986), APB (1987), Championship Sprint (Super Sprint II) (1986), Vindicators (1988), Rampage (1986), S.T.U.N. Runner (1989): Owned by Mike Mika.[1][2][4][5]
Aliens Versus Predator: Gold Edition (2000): Owned by Rebecca Ann Heineman, developer of the Mac port.[3]
San Francisco Rush: Extreme Racing (1997): Owned by M. Mika; Nintendo 64 and arcade.[4]
Doom for Atari Jaguar (1994): Owned by R. A. Heineman, partially released.[6]
Ms. Pac-Man for Sega Genesis (1991), Hard Drivin' for Sega Mega Drive (1990), Battlehawks 1942 (1988): Owned by M. Mika.[7][9]
Killing Time (1995): Owned by R. A. Heineman.[8]
Magic Johnson's Fast Break (1988), Silver Surfer (1990), Wolverine (1991): Owned by Kevin Edwards.[10][11] No plans for a public release.[12]
Heroes of Might and Magic III: Complete (2000): Owned by John Keoni Morris,[13] sent to Ubisoft.[14]
Pole Position for ZX Spectrum (1984): Owned by Graeme Devine.[15]

[1] https://x.com/MikeJMika/status/1154438658503536640
[2] https://x.com/MikeJMika/status/1154450061826248704
[3] https://x.com/burgerbecky/status/1426243074032873473
[4] https://x.com/MikeJMika/status/1444835416306171915
[5] https://x.com/MikeJMika/status/1445426867654172676
[6] https://x.com/burgerbecky/status/1436865954475134984
[7] https://x.com/MikeJMika/status/1273644318570369024
[8] https://x.com/burgerbecky/status/1434294117815705605
[9] https://x.com/MikeJMika/status/750552204034138112
[10] https://x.com/KevEdwardsRetro/status/1266512220336394244
[11] https://x.com/KevEdwardsRetro/status/1264951862404567043
[12] https://x.com/KevEdwardsRetro/status/1265013248828551168
[13] https://x.com/DiskBlitz/status/920801845349466113
[14] https://x.com/DiskBlitz/status/920803185828159490
[15] https://x.com/zaphodgjd/status/1425578432352583685