Well, if someone is comfortable with copy-write infringement or license agreement breach. Anyway, if you can test to the 20th edition then do that. Typical of AWS being out of touch with other industry, the 21st edition is not incorporated by reference and therefor not enforced by legal means....unless agreed upon by contract but who in their right mind would do that?
§192.7 What documents are incorporated by reference partly or wholly in this part?
(a) This part prescribes standards, or portions thereof, incorporated by reference into this part with the approval of the Director of the Federal Register in 5 U.S.C. 552(a) and 1 CFR part 51. The materials listed in this section have the full force of law. To enforce any edition other than that specified in this section, PHMSA must publish a notice of change in the Federal Register.
(9) API Standard 1104, “Welding of Pipelines and Related Facilities,” 20th edition, October 2005, including errata/addendum (July 2007) and errata 2 (2008), (API Std 1104), IBR approved for §§192.225(a); 192.227(a); 192.229(c); 192.241(c); and Item II, Appendix B.
If you plan on working in an area which utilizes API 1104 then it would make sense to test to the enforceable edition. If PHMSA was even close to incorporating the 21st I would suggest to go that route - but their not.