Replying to myself…

From https://rightsback.org/faq/#What_kinds_of_agreements_cannot_... , works for hire are not covered, which includes most 1990s software (though I do still wonder about open source licenses):

Under the U.S. Copyright Act, copyrighted works that qualify as “works made for hire” are subject to special rules that govern who becomes the first owner of copyright in a work. For regular works, the person who creates the work becomes the first owner of copyright. However, for “works made for hire” either the employer or person who commissioned the work becomes the first owner of copyright. Neither of these transfers of rights from the author to the employer or commissioning party, which occur by operation of the Copyright Act, nor any subsequent agreements entered into by the employer or commissioning party in relation to the work, may be challenged by the author or their family members.

There’s a whole lot more nuance there, but notably “A contribution to a collective work” is allowed to be a work for hire.