[rK] is for forms that contain kanji that are rarely used with a word. It's the lack of the okurigana "し" that would make "証人" rare, not either of the kanji.

Since this is a somewhat rare word and あかしびと【証人】 isn't in the usual refs, I think it's okay to drop. It's more likely to cause confusion with しょうにん.

@@ -6,4 +5,0 @@
-</k_ele>
-<k_ele>
-<keb>証人</keb>
-<ke_inf>&rK;</ke_inf>

証し人	1764

google news search for "証し人" (quotes) is loaded with the website "vatican news".

Parallel book title: キリストの証し人 (聖母文庫)  / The ultimate witness to Jesus Christ
https://www.amazon.co.jp/キリストの証し人-聖母文庫-フーベルト・チースリク/dp/4882161354

Reading:
http://www.catholic-takatsuki.jp/inori/index.html
神の国の証し人（あかしびと）となって

three refs in reverso. one clearly religious, two have insufficient context.  All three are "witness"
https://context.reverso.net/translation/japanese-english/証し人

https://furigana.info/w/証人
実に人は自信神に訴え自ら友と争うも力足らず、我に代りてこの事を為なす証人(あかしびと)を切に求めるのである。これ難局に処しての人間自然の要求である。

I went back to check on the "one's hand" entry.  I appreciate the stance, I just hope some of the big consumers keep up with things.

Here's a perfectly dangerous example(for example, you'd look foolish translating "witness to a crime" with this).  I don't think any other glosses are necessary.

furigana.info two examples of 証人 as あかしぶと. One is a plainly religious in context(refs). The other is from a book on aozora bunka called 炎の柱. I looked around, and it at least has 基督 with キリスト as furigana, and references クリスマス, so I'm considering that a second match.

I would not recommend merging this with 証人 しょうにん。Going on pronunciation, they are two words with occasionally-overlapping kanji forms.

I'm not sure if 証人 is truly [rK] or not, but it seems like good usage advice from what I've encountered. 証し is in sankoku, し is not optional there, which points at modern usage.  And 証人 should usually be understood as しょうにん, of course.
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Got here from 証・あかし・しょう, will have some comments there in a bit. (I encountered 証し[3] in some non-Christian, but clearly riffing on Christian ideology fiction)