Ask HN: Ones a patent expires after 10 years, does it become open source?

Patent lengths vary with the jurisdiction, and are usually more than 10 years.Inspect the laws of whatever country you are concerned with.

Whatever length, once it has expired, that method can then be used by anyone to make the item.

Whatever you make must still satisfy other legal requirements, like UL or FDA requirements in order for the item to enter into public commerce.

With expiry coming, the makers try to maintain their market exclusivity by trademark and other methods such as the need for a drug mad under the process for an illness to meet FDA approvals

The patent itself (as a document) is public domain, usually as an application before it is granted. If somebody wants to reprint, remix, or repurpose that document they are free to do.

If you want to demonstrate "reduction to practice", certainly including source code for a working implementation can shore up a patent.

Once an apple rots, does it become cider?

What is "source code" of a patent? Because "open source" concerns source code.

"Public Domain" is the term you want