I'd argue it's a common/obvious sense of 負う but I won't fight it.
I think the x-ref is unnecessary.
I don't think 深傷を負う should be sK. 深傷 isn't sK.

@@ -9 +8,0 @@
-<ke_inf>&sK;</ke_inf>
@@ -17,2 +15,0 @@
-<xref type="see" seq="1497930">負う・3</xref>
-<xref type="see" seq="1497930">負う・3</xref>

GG5, 中辞典 (both have this as examples in the 深手 entry)

I'd be happy to drop it if the receive/incur/etc. sense of 負う was the main one. It's not; we have it as #3 and some JEs have it as #5. It's obviously a standard phrase and while rhe meaning might be obvious, it's not something I'd construct from scratch. I think it's useful to show that it's used this way.

Google N-gram Corpus Counts
╭─ーーーーー─┬────────╮
│ 深手　　　 │ 18,009 │
│ 深手を負う │  1,512 │
│ 深手を負っ │  6,760 │
│ 深手を負わ │  1,581 │
│ 深手を負い │  2,398 │
╰─ーーーーー─┴────────╯

For what it's worth, 12,251 / 18,009 = roughly 70% of the usages of 深手 seem to be in this expression.
Many of the remaining usages are slight variations (e.g. 深手の傷を負う)

I still think should be deleted (see my comment from 2019). I don't think it should be kept just because it's harmless.