Incredible. The *English* wiktionary page notes the historic phonetic spelling, but not the Japanese page (this just how unimportant this is...)
https://en.wiktionary.org/wiki/山葵
Japanese
山葵 わさび Grade: 1	Jinmeiyō  jukujikun
Etymology */wasapi/ → /wasabi/
First attested in the Honzō Wamyō (918 CE) with this kanji compound and phonetic spelling of 和佐比.  

(The kanji form is not otherwise searchable on wiktionary)

No reference on the Japanese わさび page:
https://ja.wiktionary.org/wiki/わさび

Thank you, I appreciate the consideration and effort, this looks good.

Wikipedia doesn't really "have" this form.  It's not listed in the opening section as an accepted orthography. It is only noted as a historical artifact later in the article.
Headword: ワサビ（山葵[6]・山萮菜[7]、学名: Eutrema japonicum）は、...
History(名称): （918年）には、「山葵」の和名を和佐比と記している。同じく平安時代の『和名類聚抄』にも和佐比と記されている。

(山萮菜 was added randomly in 2021, I think that doesn't belong where it is either...)

和佐比 is not in nikk, it's not in daijs, it's obviously not in sankoku. I imagine it is omitted for a reason.

Sorry to repeat some numbers that are already in the notes, but:
わさび	1232009	75.6%
ワサビ	265338	16.3%
山葵	132404	8.1%
和佐比	106	0.0%

That's 106 of 1629857 occurrences, or ~1 in 15,000, or 0.0065%.  

And of those 106 occurrences, almost all are from reference materials(such as the 植物名辞典) that will have 山葵 or わさび right next to it.

The odds of encountering 和佐比 *by itself* as an attempt by an author to communicate the concept of "wasabi" seems infinitesimally small, unless one is literally studying classical Japanese texts.

Just on the numbers, this would ordinarily be an obvious [sK]. I certainly agree that [sK] shouldn't be purely a numbers game - sometimes terminology of historical/at-least-marginally useful significance is helpful to display.  This just doesn't appear to be one of those cases.

The reason that I put effort into this is that there is a cost to listing these forms in jmdict.  "Wasabi" is obviously an extremely common term. Students of Japanese will study it.  Every single person building a hand-made flashcard for わさび (paper or digital) off of this dictionary (or any of the many tools that use jmdict) has to decide for themselves if "和佐比" is worth learning(and if they lazily decide "yes", then they will waste additional time trying to learn a form that has gone mostly unused for hundreds of years).  

[rK], unfortunately, is insufficient to make this kind of choice. 磯巾着　いそぎんちゃく　"sea anemone" is [rK] at a little under 2%. It's still maybe worth learning, but rare enough that you probably should avoid it if you want a random Japanese person to understand your writing.  (This is an example I happened to be studying yesterday. 樹懶　なまけもの "tree sloth" is another fun one)

Put differently, if you write 磯巾着, a Japanese person might think you are overly educated, and/or have an unhealthy fascination with kanji (or sea anemones).

If you write 和佐比 for わさび, they will probably wonder what's the matter with you. (Since it's phonetic, they can probably guess the reading though わ　さ　ひ・び).

The amount of time wasted by thousands of individual learners adds up to something.  In exchange, what benefit does listing 和佐比 provide?  This is what is unclear to me.

Here's a big, colorful, example flashcard/study resource generated straight out of this entry(second page of a "和佐比" search)
https://www.tanoshiijapanese.com/dictionary/entry_details.cfm?entry_id=28918&element_id=201696

I'm not pressing the point beyond leaving these thoughts.  It can stay [rK] if that still feels appropriate to everyone.  But that is my motivation for "disappearing" some of these very, very rare forms.

https://ja.wikipedia.org/wiki/ワサビ#名称
ワサビの名が付く近縁な植物としてセイヨウワサビ（ホースラディッシュ）があるが、加工品の粉ワサビやチューブ入り練りワサビなどでは、原材料にセイヨウワサビのみを使用したり、両方を使っていたりするため、日本原産のワサビを本わさびと呼び、これを使ったものを高級品として区別していることが多い。

I think this is better.

@@ -28,0 +29,2 @@
+<xref type="see" seq="1121660">ホースラディッシュ</xref>
+<xref type="see" seq="2160220">西洋わさび</xref>
@@ -31 +33 @@
-<gloss>(any) blend of wasabi and horseradish</gloss>
+<gloss>horseradish (or any blend of wasabi and horseradish)</gloss>

植物名辞典 (has 和佐比), as does Wikipedia.
GG5, 中辞典, etc. all add Japanese horseradish.

Suggesting a second sense to cover Brian's issue. If this sort of information is appropriate, it shouldn't be in the literal sense.

@@ -24,0 +25,7 @@
+<gloss>Japanese horseradish</gloss>
+</sense>
+<sense>
+<pos>&n;</pos>
+<misc>&uk;</misc>
+<misc>&col;</misc>
+<gloss>(any) blend of wasabi and horseradish</gloss>