Hi, Andy -
Here is a link to the release notes for PB 2019 through PB 2022 and also a link to the release notes for PB 2017 R3, which is the Long Term Support release for 2017:
https://docs.appeon.com/pb/release_bulletin_for_pb/index.html
https://docs.appeon.com/pb2017r3/release_bulletin_for_pb/index.html
Perhaps you can find a reference to when this was added to PB.
If I may offer a suggestion: If you're going to use a not-commonly-known acronym (such as PKCE), PLEASE expand the acronym (Proof Key for Code Exchange, in this case) so that people that may be inclined to help are not inconvenienced/annoyed by having to look it up.
Also, it is helpful in many questions that get asked here to know the Release and Build numbers you are using in addition to just the PB Version number, as features are commonly added to PB in the interim releases between versions. Knowing all three numbers helps responders know if what they suggest in response to a question is appropriate for the Version, Release and Build that you are using.
Best regards, John