Fender has protected the strat design under the claim of "work of applied art" (https://spotlight.fender.com/newsroom/news/1004) which is also a concept in US copyright law, they just don't have a judgement for it etc. in their favour, unlike in Europe.

That article, and LLMs, seem to pick up on an article from US Legal Forms.[1] That article itself reads like something written by an LLM.

A more serious review of the works of applied art problem comes from the Columbia Journal of Law and the Arts.[2] That article ends with "Thus, the 'separability' line Congress has drawn, albeit often difficult to discern coherently, places most overall designs of useful articles in the public domain." Separability means being able to take the decorative design off the useful object. This covers logos on T-shirts, for example. A T-shirt with no logo still works as a T-shirt. But if you can't take the decorative part off the functional object, it's not separable. The common squiggle-shaped bicycle rack is an excellent example. That won design awards and is admired, but it's not copyrightable - you can't take the squiggle off the bike rack and still have the bike rack.[3]

The Fender Stratocaster hits that limit - take away the Strat form, and there's no guitar there.

[1] https://legal-resources.uslegalforms.com/a/applied-art-doctr...

[2] https://journals.library.columbia.edu/index.php/lawandarts/a...

[3] Brandir Int’l, Inc. v. Cascade Pac.Lumber Co https://law.justia.com/cases/federal/appellate-courts/F2/834...

I can't find the details, but this needs to be under the copyright laws of the 1950s, which were very different from today. If they didn't properly register and re-register the copyright over the years the copyright is public domain. A lawyer will need to figure out these details of course.

Edit: of course this case is in Germany, so US law doesn't apply and I claim not information on what their laws are.