!!! DEPRECATED !!! DONT USE THIS IF YOU DON'T KNOW WHAT YOURE DOING !!!

main.py

# Copyright (c) 2017 Stanislav Bobokalo & Alexey Borontov & Dominic T

# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the "Software"), to deal
# in the Software without restriction, including without limitation the rights
# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
# copies of the Software, and to permit persons to whom the Software is
# furnished to do so, subject to the following conditions:

# The above copyright notice and this permission notice shall be included in all
# copies or substantial portions of the Software.

# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
# SOFTWARE.

# !!! DEPRECATED !!! DONT USE THIS IF YOU DON'T KNOW WHAT YOURE DOING !!!
# Take a look here: https://github.com/gurland/telegram-delete-all-messages

from time import sleep
from os import getenv

from pyrogram import Client
from pyrogram.raw.functions.messages import Search
from pyrogram.raw.types import InputPeerSelf, InputMessagesFilterEmpty
from pyrogram.raw.types.messages import ChannelMessages
from pyrogram.errors import FloodWait, UnknownError


API_ID = getenv('API_ID', None) or int(input('Enter your Telegram API id: '))
API_HASH = getenv('API_HASH', None) or input('Enter your Telegram API hash: ')
PHO_NMBR = getenv('PHONE_NUMBER', None) or input('Enter your Phone number: ')

app = Client("client", api_id=API_ID, api_hash=API_HASH, phone_number=PHO_NMBR)
app.start()


class Cleaner:
    def __init__(self, peer=None, chat_id=None):
        self.peer = peer
        self.chat_id = chat_id
        self.message_ids = []
        self.add_offset = 0

    def select_supergroup(self):
        dialogs = app.get_dialogs()
        groups = [x for x in dialogs if x.chat.type == 'supergroup' or x.chat.type == 'group']

        for i, group in enumerate(groups):
            print(f'{i+1}. {group.chat.title}')

        print('')

        group_n = int(input('Insert group number: '))
        selected_group = groups[group_n - 1]

        selected_group_peer = app.resolve_peer(selected_group.chat.id)
        self.peer = selected_group_peer
        self.chat_id = selected_group.chat.id

        print(f'Selected {selected_group.chat.title}\n')

        return selected_group, selected_group_peer

    def run(self):
        q = self.search_messages()
        self.update_ids(q)
        messages_count = len(q.messages)
        print(f'Found {messages_count} your messages in selected supergroup')

        if messages_count < 100:
            pass
        else:
            self.add_offset = 100

            for i in range(0, messages_count, 100):
                q = self.search_messages()
                self.update_ids(q)
                self.add_offset += 100

        self.delete_messages()

    @staticmethod
    def chunks(l, n):
        """Yield successive n-sized chunks from l.
        https://stackoverflow.com/questions/312443/how-do-you-split-a-list-into-evenly-sized-chunks#answer-312464"""
        for i in range(0, len(l), n):
            yield l[i:i + n]

    def update_ids(self, query: ChannelMessages):
        for msg in query.messages:
            self.message_ids.append(msg.id)

        return len(query.messages)

    def delete_messages(self):
        print(f'Deleting {len(self.message_ids)} messages with next message IDs:')
        print(self.message_ids)
        for message_ids_chunk in self.chunks(self.message_ids, 100):
            try:
                app.delete_messages(chat_id=self.chat_id, message_ids=message_ids_chunk)
            except FloodWait as flood_exception:
                sleep(flood_exception.x)

    def search_messages(self):
        print(f'Searching messages. OFFSET: {self.add_offset}')
        return app.send(
            Search(
                peer=self.peer,
                q='',
                filter=InputMessagesFilterEmpty(),
                min_date=0,
                max_date=0,
                offset_id=0,
                add_offset=self.add_offset,
                limit=100,
                max_id=0,
                min_id=0,
                hash=0,
                from_id=InputPeerSelf()
            )
        )


if __name__ == '__main__':
    try:
        deleter = Cleaner()
        deleter.select_supergroup()
        deleter.run()
    except UnknownError as e:
        print(f'UnknownError occured: {e}')
        print('Probably API has changed, ask developers to update this utility')
    finally:
        app.stop()

@Magnuswlange please notice that this will only work for "group" and "supergroup" chats. Not for channels or private.
If you would need these features, I would recommend using this instead: https://github.com/gurland/telegram-delete-all-messages

Sadly I dont have the time to keep this fixxed and updated.
I just published it because it worked for me.

I understand, thank you. The issues I were having with the one you linked, was that it wasn't working for 1 specific group, hence I tried your's. Can't have any of them clear 1 specific channel for me. Thank you anyways.