Unknown height system (US Feet) - ESRI:115808

WKT

VERTCS["Unknown_height_system_(US_survey_feet)", 
	VDATUM["Unknown_height_system_(US_survey_feet)"], 
	PARAMETER["Vertical_Shift", 0.0], 
	PARAMETER["Direction", 1.0], 
	UNIT["Foot_US", 0.3048006096012192]]

WKT2

VERTCRS["Unknown_height_system_(US_survey_feet)", 
	VDATUM["Unknown_height_system_(US_survey_feet)"], 
	CS[vertical, 1], 
	AXIS["Gravity-related height (H)", up, 
		LENGTHUNIT["Foot_US", 0.3048006096012192]], 
	ID["Esri", "115808"]]

Data from @esri/proj-codes

{
  "wkid": 115808,
  "latestWkid": 115808,
  "macro": "PE_VCS_UNKNOWN_HT_SYSTEM_USFT",
  "name": "Unknown_height_system_(US_survey_feet)",
  "wkt": "VERTCS[\"Unknown_height_system_(US_survey_feet)\",VDATUM[\"Unknown_height_system_(US_survey_feet)\"],PARAMETER[\"Vertical_Shift\",0.0],PARAMETER[\"Direction\",1.0],UNIT[\"Foot_US\",0.3048006096012192]]",
  "wkt2": "VERTCRS[\"Unknown_height_system_(US_survey_feet)\",VDATUM[\"Unknown_height_system_(US_survey_feet)\"],CS[vertical,1],AXIS[\"Gravity-related height (H)\",up,LENGTHUNIT[\"Foot_US\",0.3048006096012192]],ID[\"Esri\",\"115808\"]]",
  "description": "Unknown height system (US Feet)",
  "authority": "Esri",
  "version": "10.5.1",
  "deprecated": "no",
  "areaname": "World",
  "extent": {
    "slat": -90,
    "nlat": 90,
    "llon": -180,
    "rlon": 180
  }
}