I think this is a very confusing idea, but I wondered if the copyright has an ending date.
I do know from my late FIL, who patented plants, that his patents had a life span of about 17 years, after that anyone could take cuttings, grow and sell those same plants under any name they choose to put on them. That was in the 1960s, I think now it's 20 years.
But intellectual property seems to be far different. This is what I found on the Internet.
On the other hand, this doesn't bother me since I find it difficult to follow intricate patterns and much prefer to make up my own, or some I find in old family pictures. Those I suppose could be legally "mine" if I would care to do something about them, but they look a lot like most you see now so there's no problem about doing something like them.
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How long does a copyright last? In 1998, Congress fine-tuned the law to allow works to be copyrighted for the life of the creator plus 70 years. This means that 70 years after the creator dies, the copyright expires if no family heir files for an extension to renew it. After that it is in “Public Domain,” allowing anyone to use the work. So the fact that a magazine, book or pattern is out of print, or the author is dead, does not mean you can copy it.
(Or at least until it is so old no one could read the printing?)