This requires lcu_driver from https://github.com/sousa-andre/lcu-driver to work, if it doesn't work you're probably missing that requirement.

LeagueFriendsCleaner.py

from lcu_driver import Connector
import sys, time

# WARNING: THIS WILL START CLEANING WHEN YOU RUN IT!
# For friends you want to keep, just add any note to them.
# You can also put them in a category called "Friends".

def out(string):
	print(string, flush=True)

conn = Connector()

# Fired when LCU API is ready to be used
@conn.ready
async def connect(conn):
	out(f'Nuking Friends List!')
	
	friendsList = await conn.request('get', '/lol-chat/v1/friends')
	friendsList = await friendsList.json()

	rates = 0

	for friend in friendsList:
		if friend['groupName'] == "Friends":
			continue
		if friend['note'] != "":
			continue
		
		sid = friend['id']
		
		sys.stdout.buffer.write(f"REMOVING: {friend['name']}\n".encode('utf8'))
		data = await conn.request('delete', f'/lol-chat/v1/friends/{sid}')
		data = await data.json()
		out(data)
		
		time.sleep(0.5)
		rates += 1

		if rates > 9:
			rates = 0
			time.sleep(1)


# Fired when League Client is closed (or disconnected from websocket)
@conn.close
async def disconnect(_):
	await conn.stop()
	quit()

# starts the connector
conn.start()

This script isn’t endorsed by Riot Games and doesn’t reflect the views or opinions of Riot Games or anyone officially involved in producing or managing League of Legends. League of Legends and Riot Games are trademarks or registered trademarks of Riot Games, Inc. League of Legends © Riot Games, Inc.