Is there a way to make a chat filter bot detect capital letters through code? (discord.py)

This should work. I've tried to stay as close to your original code as possible.

import discord
import random
import string
from discord.ext import commands

client = commands.Bot(command_prefix = 'as-')

with open("badwords2.txt") as file:
    bad_words = file.read().splitlines()
    
def check_if_allowed(text):

    allowed_characters = string.ascii_uppercase + string.ascii_lowercase #creates a string containing all upper- and lower- case letters.

    allowed_characters += "1234567890 " # adds digits to the string

    for character in text: #iterates through every character in the input
        if character not in allowed_characters: #checks if the character is in the set of allowed characters, and returns false if not
            return False

    return True #returns true if every character in the input is allowed
    
@client.event
async def on_message(message):
    if not check_if_allowed(message.content):
        t = discord.Embed(color=0x039e00, title="Message Removed", description=":x:  Please only use alpha-numeric characters and spaces.")
        t.set_footer(text="DM TheSuperRobert2498#2498 for bot suggestions.")
        await message.channel.send(embed=t)
        await message.delete()
        return
    
    for bad_word in bad_words:
        if bad_word in message.content.lower().split(" "):
            t = discord.Embed(color=0x039e00, title="Message Removed", description=":x:   Please do not swear. Swearing can result in a mute or ban.")
            t.set_footer(text="DM TheSuperRobert2498#2498 for bot suggestions.")
            await message.channel.send(embed=t)
            await message.delete()
            return


client.run('TOKEN')