Is pasting an excerpt from Wikipedia in a paper without reference plagiarism?

Copyright for a written work expires some years after the author's death; the length of time under copyright law varies from country to country. Wikipedia and other sites use a variety of licencing models which allow the material to be copied and the conditions that apply to the use of the material.

But regardless of whether something is still copyrighted or not, open source or not, freely available or not, if it is quoted, cited, copied or otherwise repeated in a text, if the source is not cited then yes, it is plagiarism.

When something passes into "common knowledge" then it can be written without having to find a quotable source. But there are also specific situations where something that might normally be considered "common knowledge" needs to be referenced to a source, such as student work where values, definitions, etc., are expected to be researched for accuracy and should also therefore include a reference.

There may also be the case where the value/definition is used in a way that is outside the norm, e.g. a definition that is unusual in the mainstream but is used in a specific way for a specific context, or a value that is based on some work that extends the precision beyond what is considered to be the norm.

The criteria remains, that if someone else wrote it and you copy it then not crediting the original author/source is plagiarism - academic theft.

Yes, that is plagiarism. But even beyond that, it's not true that

Wikipedia is freely available and anonymous

Instead, as you will find on the bottom of every Wikipedia article,

Text is available under the Creative Commons Attribution-ShareAlike License; additional terms may apply.

That license specifically requires (as by its short summary)

Attribution—You must attribute the work in the manner specified by the author or licensor (but not in any way that suggests that they endorse you or your use of the work.)

Wikipedia has a very lenient understanding of proper attribution – normally, attribution according to the CC licenses is the extreme opposite of anonymous! Traditionally, a CC license would require listing every single contributor to the document used. The revision history of a Wikipedia article (I'll use ‘Eigenvalues and eigenvectors’ as example) is public, and statistics suggest that most of the edits are not anonymous (IPs), but at least pseudonymous (users with named accounts) and several of the main authors have account names that suggest those are their real names. We are permitted to give attribution to “the Wikipedia contributors“ (which is at least somewhat anonymous) instead of that list, but an appropriate mention is still required, such as

Wikipedia contributors. Eigenvalues and eigenvectors. Wikipedia, The Free Encyclopedia. August 3, 2018, 15:11 UTC. Available at: https://en.wikipedia.org/w/index.php?title=Eigenvalues_and_eigenvectors&oldid=853269180. Accessed August 8, 2018.

(In addition, that license means that if someone makes use of a text under this license, for example, using some definition on Wikipedia in their paper, they would have to publish their document under a compatible license!)

Yes, it's plagiarism. Being freely available and anonymous doesn't mean you can copy from it liberally, because it's still written by someone other than yourself.

Note that Wikipedia has its own "cite this page" link (example), indicating it also thinks it should be cited if you take information from it.