According to USPTO trademark search, WordStar trademark is dead (cancelled on August 14, 1995). This is US-only, of course.

The trademark maybe dead but the product is not abandoned, nor has it been released into the public domain. The current owners are Houghton Mifflin Harcourt:

https://en.wikipedia.org/wiki/Houghton_Mifflin_Harcourt

I'm sure if they felt like it they could assert their IP rights over any clones or similarly named products.

What IP rights?

The trademark is dead. That's naming.

Patents? Seems unlikely.

That leaves copyright. It seems unlikely that anyone working on a Wordstar clone is silly enough to have even glanced at the original source, so that's not it.

Look-and-feel? Dead issues. And there's no API to copy, not that I believe an API is copyrightable as it is a functional spec. The United States Court of Appeals for the Federal Circuit can bite me.