Example that uses ArcGIS Python API to authenticate, but uses Requests to make calls to the backend REST service directly.

delete_AGOL_FS_Features.py

from arcgis.gis import GIS
import requests
import json

USER = "USER"
PASS = "PASSWORD"

g = GIS("https://www.arcgis.com", USER, PASS)

token = g._con.token

payload = {"where": "1=1",
			"f":"json",
			"token": token}

url = "https://services1.arcgis.com/orgID/ArcGIS/rest/services/p5000/FeatureServer/0"

p = {**payload, **{"returnCountOnly": "true"}}
response = requests.post(url + "/query", data = p)
r = json.loads(response.text)
print("Count before: {}".format(r))

p = {**payload, **{"async": "false"}}
response = requests.post(url + "/truncate", data = p)
r = json.loads(response.text)
print("Result of delete: {}".format(r))


p = {**payload, **{"returnCountOnly": "true"}}
response = requests.post(url + "/query", data = p)
r = json.loads(response.text)
print("Count after: {}".format(r))