Captain Beyond

The public domain is a concept in copyright law where a work has no copyright holder, either because the copyright expired or because (in TempleOS’s case) the copyright was voluntarily relinquished. However, note that it’s entirely jurisdiction-dependent (for example, it’s not always even possible to relinquish a copyright, and copyright expiration also varies by jurisdiction). As copyright is a state-granted monopoly on distribution rights, if no one has that monopoly, there are no restrictions on copying and hence the freedom to modify and distribute the source code is available to all. Note that copyright is automatic upon the creation of a work and does not need to be registered, so either an explicit declaration from the author or knowledge that the copyright term is expired is needed to know something is in the public domain.

In short, if something is in the public domain (as the preferred form for modification i.e. source code) it is free software and open source, but the idea that public domain is somehow incompatible with FOSS is a somewhat reoccurring talking point that came up very recently. I don’t know where this came from but I imagine it has something to do with the discourse on copyright on LLM generated work.

However, it’s usually best to use an explicit license so you can affirm your intent to release the software as free.

FSF recognizes public domain as a category of free software (although, in their pedantry, they are quick to point out that it is not a “license”):

https://www.gnu.org/licenses/license-list.html#PublicDomain

OSI says more or less the same thing although they also elaborate on the actual legal complications of the public domain, and recommend explicitly using a license if possible:

https://opensource.org/faq#public-domain