Here is the result of some online research.
This post is about the copyright for the original RWS tarot only, not for alterations made by US Games like new coloring, new titles, tarot boxes, trademarks, etc. The RWS tarot was first published in the UK in 1909. The Pictorial Key to the Tarot with black and white line drawings was first published in 1911. The artwork was a hired work by Pamela Coleman Smith for R. A. Waite. Waite died in 1942, Smith died in 1951.
RWS-copyright in the UK
The duration of copyright at the time the RWS was published was 42 years from the date of publication, according to the 1842 copyright act. By applying this law the RWS would have been in the public domain since the beginning of 1953, the Pictorial Key since 1955.
Later implementations, the 1911 act (holding for work published from July, 1st 1912) and 1988 act in particular, would only apply if their rules about copyright duration were to be treated as retroactive (that means applying to cases from before the implementation of the law). Most online sources I found seem to assume this, but since British law, as far as I know, is generally not retroactive unless stated otherwise, there should be some regulations or parts of the law saying so explicitly. There is for example a regulation from 1996 stating in which cases the new copyright duration rule would apply (
http://www.legislation.gov.uk/uksi/1995/3297/regulation/16/made). If the RWS copyright would still have existed in any other EU countries at the end of 1995 then the extended copyright duration from the 1988 law would be revived in the UK as well. There might also be other reasons for the extension of the copyright period that I have missed so far. This would mean 70 years from the author’s death, and the RWS would only have been in the public domain since the beginning of 2013.
There is a claim by US Games that since the RWS tarot consisted of a joint authorship of Smith and Waite the copyright duration would last for 70 years past the year Smith died instead of the year Waite died. By applying the 1911 copyright act this would be true as hired work in this act counts as a joint authorship as long as the hired person worked as a freelancer. The RWS copyright would only expire in 2021. However this claim is valid only if the joint authorship rule in the 1911 act would have to be applied retroactively as well. I have found no indication for that.
Conclusion: It is most likely that the copyright for the RWS/Pictoral Key expired at the end of 2012. Assuming that the copyright will only expire in 2021 is much less likely. The facts I found are not enough to be certain though.
ETA (correction):: British copyright law appears to be retroactive. That means RWS copyright expires in 2021. See post #38 in this thread.
RWS-copyright in the EU
Due to several international agreements the same rules as in the UK apply.
RWS-copyright in the US
The UK copyright duration has no relevance for the US.
“Under current American law, both domestic and foreign publications published prior to 1923 are in the public domain.” (
http://www.dlib.org/dlib/july08/hirtle/07hirtle.html). Some exceptions are mentioned here (
http://copyright.cornell.edu/resources/publicdomain.cfm) none of which apply to the RWS tarot. This view is also held by Wikipedia (
https://en.wikipedia.org/wiki/Rider-Waite_tarot_deck). There is no ambiguity in any of the online sources I found.
However, US Games has made a couple of claims to argue in favor of a longer copyright duration for the RWS. These claims are described in more detail on the Sacred Texts site (
http://www.sacred-texts.com/tarot/faq.htm).
The GATT allows copyright restoration for foreign works under a number of circumstances. This only applies if, among other conditions to be fulfilled, “The work was in the public domain in the US because the work did not comply with formalities imposed at any time by the US law” (
http://www.publicdomainsherpa.com/copyright-restoration.html). I don’t think that this applies to work prior to 1923 anyway. Regardless of that US Games has successfully claimed US copyright for the RWS, therefore copyright restoration does not apply.
US Games also refers to the Berne convention, which contains a copyright protection for foreign work of 50 years past the author’s death. When the UK later extended the copyright duration to 70 years this did not matter in the US because of the “rule of the shorter term” (
https://en.wikipedia.org/wiki/Berne_Convention#Copyright_term) and the Berne convention not being retroactive.
Conclusion: There is no doubt that the RWS and Pictoral Key have been in the public domain in the US long since.