ISN2016 + ISH2004 height - EPSG:9950

WKT

HVCOORDSYS["ISN2016_and_ISH2004_height", 
	GEOGCS["ISN2016", 
		DATUM["Islands_Net_2016", 
			SPHEROID["GRS_1980", 6378137.0, 298.257222101]], 
		PRIMEM["Greenwich", 0.0], 
		UNIT["Degree", 0.0174532925199433]], 
	VERTCS["ISH2004_height", 
		VDATUM["Landshaedarkerfi_Islands_2004"], 
		PARAMETER["Vertical_Shift", 0.0], 
		PARAMETER["Direction", 1.0], 
		UNIT["Meter", 1.0]]]

WKT2

COMPOUNDCRS["ISN2016_and_ISH2004_height", 
	GEOGCRS["ISN2016", 
		DATUM["Islands_Net_2016", 
			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["ISH2004_height", 
		VDATUM["Landshaedarkerfi_Islands_2004"], 
		CS[vertical, 1], 
		AXIS["Gravity-related height (H)", up, 
			LENGTHUNIT["Meter", 1.0]]], 
	ID["EPSG", "9950"]]

Data from @esri/proj-codes

{
  "wkid": 9950,
  "latestWkid": 9950,
  "pcsWkid": 0,
  "gcsWkid": 8086,
  "vcsWkid": 8089,
  "macro": "PE_HVC_ISN2016_ISH2004_HEIGHT",
  "name": "ISN2016_and_ISH2004_height",
  "wkt": "HVCOORDSYS[\"ISN2016_and_ISH2004_height\",GEOGCS[\"ISN2016\",DATUM[\"Islands_Net_2016\",SPHEROID[\"GRS_1980\",6378137.0,298.257222101]],PRIMEM[\"Greenwich\",0.0],UNIT[\"Degree\",0.0174532925199433]],VERTCS[\"ISH2004_height\",VDATUM[\"Landshaedarkerfi_Islands_2004\"],PARAMETER[\"Vertical_Shift\",0.0],PARAMETER[\"Direction\",1.0],UNIT[\"Meter\",1.0]]]",
  "wkt2": "COMPOUNDCRS[\"ISN2016_and_ISH2004_height\",GEOGCRS[\"ISN2016\",DATUM[\"Islands_Net_2016\",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[\"ISH2004_height\",VDATUM[\"Landshaedarkerfi_Islands_2004\"],CS[vertical,1],AXIS[\"Gravity-related height (H)\",up,LENGTHUNIT[\"Meter\",1.0]]],ID[\"EPSG\",\"9950\"]]",
  "description": "ISN2016 + ISH2004 height",
  "authority": "EPSG",
  "version": "10.056(13.2.0)",
  "deprecated": "no",
  "areaname": "Iceland - onshore",
  "extent": {
    "slat": 63.24,
    "nlat": 66.62,
    "llon": -24.66,
    "rlon": -13.38
  }
}