As discussed in the いたく entry, we're dropping sense 3 altogether.

@@ -30,6 +29,0 @@
-</sense>
-<sense>
-<pos>&adj-i;</pos>
-<misc>&uk;</misc>
-<s_inf>rare except as いたく</s_inf>
-<gloss>exceeding</gloss>

Trimming back sense 3. No sign it's "arch"; there's even an example sentence using it. Dropping the xref as I propose dropping the target entry.

@@ -33,2 +32,0 @@
-<xref type="see" seq="1369990">痛く</xref>
-<xref type="see" seq="1369990">痛く</xref>
@@ -36,2 +34 @@
-<misc>&arch;</misc>
-<s_inf>rare except as いたく・痛く・甚く</s_inf>
+<s_inf>rare except as いたく</s_inf>

This might be one of the proportionally rarest kanji forms I've ever seen
痛い	9757936	100.0%
甚い	110	0.0%

But as [3] (and only as [3]), this gets some use:
甚く	4732	0.0%

Big note: sankoku makes a dedicated entry for 甚く・痛く, and does *not* include sense [3] under 痛い at all. Highly unusual for an adjective. [3] may be [arch] as [adj-i].

Can't say "esp. 甚い" on [3], since [3] is [uk], and 痛く is still more common than 甚く.  Fair enough that [3] is pretty much the only place 甚 is used． As daijs notes, when it is used it is not is いたい, but as いたく, which we have a separate entry for(where 甚い is not [rK]).

５ （甚い）程度のはなはだしいさま。＊＊＊＊＊多く、連用形を用いる。→甚(いた)く＊＊＊＊＊＊

Curious how [suf] should apply to [3].  We don't use [suf] for adjectives like 深い (興味深い, and 20+ similar entries).  Seems to be reserved for things like にくい、やすい、etc. that modify verb stems (食べやすい、読みにくい) or くさい with [id] meanings ("smells-like"/seems-like).  I don't see how [3] could possibly do this...)

===

Aside: strong agree with Marcus from some years back on [int] "ouch!".  I can't think of a more natural Japanese interjection than that.
https://japaneseparticlesmaster.xyz/itai-in-japanese/
but can also work as an interjection, especially in speaking, to mean ‘ouch’ in Japanese.

https://eow.alc.co.jp/search?q=痛い
痛い！
間投
ouch
ow〔突然の痛み〕

https://context.reverso.net/translation/japanese-english/痛い#ouch

@@ -11,0 +12 @@
+<ke_inf>&rK;</ke_inf>
@@ -32,4 +33,4 @@
-<pos>&suf;</pos>
-<xref type="see" seq="1369990">甚く</xref>
-<xref type="see" seq="1369990">甚く</xref>
-<s_inf>esp. 甚い</s_inf>
+<xref type="see" seq="1369990">痛く</xref>
+<misc>&uk;</misc>
+<misc>&arch;</misc>
+<s_inf>rare except as いたく・痛く・甚く</s_inf>

wiktionary
"今すぐやめて！SNSで「イタい女」認定されちゃう投稿まとめ"
https://matome.naver.jp/odai/2149878833978295001

@@ -25,0 +26,6 @@
+<misc>&col;</misc>
+<gloss>cringy</gloss>
+<gloss>embarrassing</gloss>
+</sense>
+<sense>
+<pos>&adj-i;</pos>