ETRS89 + ODN Orkney height - EPSG:9426

WKT

HVCOORDSYS["ETRS_1989_and_ODN_Orkney_height", 
	GEOGCS["GCS_ETRS_1989", 
		DATUM["D_ETRS_1989", 
			SPHEROID["GRS_1980", 6378137.0, 298.257222101]], 
		PRIMEM["Greenwich", 0.0], 
		UNIT["Degree", 0.0174532925199433]], 
	VERTCS["Newlyn_Orkney_Isles", 
		VDATUM["Ordnance_Datum_Newlyn_Orkney_Isles"], 
		PARAMETER["Vertical_Shift", 0.0], 
		PARAMETER["Direction", 1.0], 
		UNIT["Meter", 1.0]]]

WKT2

COMPOUNDCRS["ETRS_1989_and_ODN_Orkney_height", 
	GEOGCRS["GCS_ETRS_1989", 
		DATUM["D_ETRS_1989", 
			ELLIPSOID["GRS_1980", 6378137.0, 298.257222101]], 
		PRIMEM["Greenwich", 0.0], 
		CS[ellipsoidal, 2], 
		AXIS["Latitude (lat)", north, 
			ORDER[1]], 
		AXIS["Longitude (lon)", east, 
			ORDER[2]], 
		ANGLEUNIT["Degree", 0.0174532925199433]], 
	VERTCRS["Newlyn_Orkney_Isles", 
		VDATUM["Ordnance_Datum_Newlyn_Orkney_Isles"], 
		CS[vertical, 1], 
		AXIS["Gravity-related height (H)", up, 
			LENGTHUNIT["Meter", 1.0]]], 
	ID["EPSG", "9426"]]

Data from @esri/proj-codes

{
  "wkid": 9426,
  "latestWkid": 9426,
  "pcsWkid": 0,
  "gcsWkid": 4258,
  "vcsWkid": 5740,
  "macro": "PE_HVC_ETRS89_ODN_ORKNEY_HEIGHT",
  "name": "ETRS_1989_and_ODN_Orkney_height",
  "wkt": "HVCOORDSYS[\"ETRS_1989_and_ODN_Orkney_height\",GEOGCS[\"GCS_ETRS_1989\",DATUM[\"D_ETRS_1989\",SPHEROID[\"GRS_1980\",6378137.0,298.257222101]],PRIMEM[\"Greenwich\",0.0],UNIT[\"Degree\",0.0174532925199433]],VERTCS[\"Newlyn_Orkney_Isles\",VDATUM[\"Ordnance_Datum_Newlyn_Orkney_Isles\"],PARAMETER[\"Vertical_Shift\",0.0],PARAMETER[\"Direction\",1.0],UNIT[\"Meter\",1.0]]]",
  "wkt2": "COMPOUNDCRS[\"ETRS_1989_and_ODN_Orkney_height\",GEOGCRS[\"GCS_ETRS_1989\",DATUM[\"D_ETRS_1989\",ELLIPSOID[\"GRS_1980\",6378137.0,298.257222101]],PRIMEM[\"Greenwich\",0.0],CS[ellipsoidal,2],AXIS[\"Latitude (lat)\",north,ORDER[1]],AXIS[\"Longitude (lon)\",east,ORDER[2]],ANGLEUNIT[\"Degree\",0.0174532925199433]],VERTCRS[\"Newlyn_Orkney_Isles\",VDATUM[\"Ordnance_Datum_Newlyn_Orkney_Isles\"],CS[vertical,1],AXIS[\"Gravity-related height (H)\",up,LENGTHUNIT[\"Meter\",1.0]]],ID[\"EPSG\",\"9426\"]]",
  "description": "ETRS89 + ODN Orkney height",
  "authority": "EPSG",
  "version": "9.8.13(13.2.0)",
  "deprecated": "no",
  "areaname": "UK - Orkney Islands onshore",
  "extent": {
    "slat": 58.72,
    "nlat": 59.41,
    "llon": -3.48,
    "rlon": -2.34
  }
}