BGS2005 height - EPSG:9669

WKT

VERTCS["BGS2005_height", 
	VDATUM["Bulgarian_Height_System_2005"], 
	PARAMETER["Vertical_Shift", 0.0], 
	PARAMETER["Direction", 1.0], 
	UNIT["Meter", 1.0]]

WKT2

VERTCRS["BGS2005_height", 
	VDATUM["Bulgarian_Height_System_2005"], 
	CS[vertical, 1], 
	AXIS["Gravity-related height (H)", up, 
		LENGTHUNIT["Meter", 1.0]], 
	ID["EPSG", "9669"]]

Data from @esri/proj-codes

{
  "wkid": 9669,
  "latestWkid": 9669,
  "macro": "PE_VCS_BGS2005_HT",
  "name": "BGS2005_height",
  "wkt": "VERTCS[\"BGS2005_height\",VDATUM[\"Bulgarian_Height_System_2005\"],PARAMETER[\"Vertical_Shift\",0.0],PARAMETER[\"Direction\",1.0],UNIT[\"Meter\",1.0]]",
  "wkt2": "VERTCRS[\"BGS2005_height\",VDATUM[\"Bulgarian_Height_System_2005\"],CS[vertical,1],AXIS[\"Gravity-related height (H)\",up,LENGTHUNIT[\"Meter\",1.0]],ID[\"EPSG\",\"9669\"]]",
  "description": "BGS2005 height",
  "authority": "EPSG",
  "version": "10.013(12.9.0)",
  "deprecated": "no",
  "areaname": "Bulgaria - onshore",
  "extent": {
    "slat": 41.24,
    "nlat": 44.23,
    "llon": 22.36,
    "rlon": 28.68
  }
}