Caspian height - EPSG:5611

WKT

VERTCS["Caspian_height", 
	VDATUM["Caspian_Sea"], 
	PARAMETER["Vertical_Shift", 0.0], 
	PARAMETER["Direction", 1.0], 
	UNIT["Meter", 1.0]]

WKT2

VERTCRS["Caspian_height", 
	VDATUM["Caspian_Sea"], 
	CS[vertical, 1], 
	AXIS["Gravity-related height (H)", up, 
		LENGTHUNIT["Meter", 1.0]], 
	ID["EPSG", "5611"]]

Data from @esri/proj-codes

{
  "wkid": 5611,
  "latestWkid": 5611,
  "macro": "PE_VCS_CASPIAN_HEIGHT",
  "name": "Caspian_height",
  "wkt": "VERTCS[\"Caspian_height\",VDATUM[\"Caspian_Sea\"],PARAMETER[\"Vertical_Shift\",0.0],PARAMETER[\"Direction\",1.0],UNIT[\"Meter\",1.0]]",
  "wkt2": "VERTCRS[\"Caspian_height\",VDATUM[\"Caspian_Sea\"],CS[vertical,1],AXIS[\"Gravity-related height (H)\",up,LENGTHUNIT[\"Meter\",1.0]],ID[\"EPSG\",\"5611\"]]",
  "description": "Caspian height",
  "authority": "EPSG",
  "version": "9.8.12(10.0.0)",
  "deprecated": "no",
  "areaname": "Asia - FSU - Caspian Sea",
  "extent": {
    "slat": 37.35,
    "nlat": 46.97,
    "llon": 46.95,
    "rlon": 53.93
  }
}