It seems CC0 may have simplified this now. The issue used to be that software enters the public domain after copyright expires & different countries interpret that different ways, which can become complicated without an explicit license document.

“Using CC0 for Public Domain Software” https://creativecommons.org/2011/04/15/using-cc0-for-public-...

“CC0 FAQ” https://wiki.creativecommons.org/wiki/CC0_FAQ#May_I_apply_CC...