Regarding GNU diffutils, the code that was copied is the shift_boundaries function. git blame on 2.7 of diffutils shows that it was written by David MacKenzie and Paul Eggert - we could try reaching out to both of them to see if they're interested in relicensing?

In T196132#4248047, @Legoktm wrote:

Regarding GNU diffutils, the code that was copied is the shift_boundaries function. git blame on 2.7 of diffutils shows that it was written by David MacKenzie and Paul Eggert - we could try reaching out to both of them to see if they're interested in relicensing?

Sure. I suppose it's Eggert at UCLA: http://web.cs.ucla.edu/classes/spring09/cs111/mail-eggert.html

Is the copyright with David MacKenzie himself? According to https://www.gnu.org/people/people.html#d, he worked for FSF, so he probably signed a copyright assignment, right? https://www.fsf.org/bulletin/2014/spring/copyright-assignment-at-the-fsf Better ask him first, but FSF/GNU may also be able to help.

In addition to shift_boundaries, compareseq is also quite similar to the one in diffutils. But although diffutils has a diag, Dairiki's diag is quite different and is apparently his own work. He cited two other sources as inspiration (NEDKONZ Algorithm::Diff and lecture notes by David Eppstein) but neither has code that is especially similar to Dairiki's diag or its helper lcs_pos. The LCS interpretation code in diff is also apparently Dairiki's own work.

Dairiki's difflib.php carried the following copyright header:

// Copyright (C) 2000, 2001 Geoffrey T. Dairiki <dairiki@dairiki.org>
// You may copy this code freely under the conditions of the GPL.

diffutils's analyze.c carries the following copyright header:

Copyright (C) 1988, 1989, 1992, 1993 Free Software Foundation, Inc.

Note that Dairiki's code also appears in PEAR Text_Diff, with an LGPL license, and is said to be "used/adapted with his permission". So he probably already agreed to license it as LGPL.

He's active on GitHub.