ETRF91 - ESRI:104180

WKT

GEOGCS["ETRF91", 
	DATUM["European_Terrestrial_Reference_Frame_1991", 
		SPHEROID["GRS_1980", 6378137.0, 298.257222101]], 
	PRIMEM["Greenwich", 0.0], 
	UNIT["Degree", 0.0174532925199433]]

WKT2

GEOGCRS["ETRF91", 
	DYNAMIC[
		FRAMEEPOCH[1989.0], 
		MODEL["NNR-NUVEL1"]], 
	DATUM["European_Terrestrial_Reference_Frame_1991", 
		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], 
	ID["Esri", "9061"]]

Data from @esri/proj-codes

{
  "wkid": 104180,
  "latestWkid": 9061,
  "macro": "PE_GCS_ETRF_1991",
  "name": "ETRF91",
  "wkt": "GEOGCS[\"ETRF91\",DATUM[\"European_Terrestrial_Reference_Frame_1991\",SPHEROID[\"GRS_1980\",6378137.0,298.257222101]],PRIMEM[\"Greenwich\",0.0],UNIT[\"Degree\",0.0174532925199433]]",
  "wkt2": "GEOGCRS[\"ETRF91\",DYNAMIC[FRAMEEPOCH[1989.0],MODEL[\"NNR-NUVEL1\"]],DATUM[\"European_Terrestrial_Reference_Frame_1991\",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],ID[\"Esri\",\"9061\"]]",
  "description": "ETRF91",
  "authority": "Esri",
  "version": "10.6.0",
  "deprecated": "codechange",
  "areaname": "Europe - ETRF",
  "extent": {
    "slat": 33.26,
    "nlat": 84.73,
    "llon": -16.1,
    "rlon": 38.01
  },
  "frameEpoch": 1989,
  "deformationModel": "NNR-NUVEL1"
}