Start Steam, forcing offline mode without a prompt. Save in your Steam folder, "C:\Program Files (x86)\Steam" on x86_64 Windows.

Steam.py

import os

USERFILE = "config/loginusers.vdf"
OFFLINE_KEYS = ["WantsOfflineMode", "SkipOfflineModeWarning"]

with open(USERFILE, "r") as f:
    lines = f.readlines()

with open(USERFILE, "w") as f:
    nest = 0

    for line in lines:
        key = line.strip().split()[0].strip('"')

        # skip writing the existing (bad) values of keys related
        # to offline functionality
        if key in OFFLINE_KEYS:
            continue

        # keep track of nesting level
        if key == "{":
            nest += 1

        # if we're closing a user section, write correct values
        # to the user object
        if key == "}":
            if nest == 2:
                for offline_key in OFFLINE_KEYS:
                    f.write("\t" * nest)
                    f.write(f"\"{offline_key}\"\t\"1\"\n")
            nest -= 1

        f.write(line)

os.execl("Steam.exe", "Steam.exe")

https://pypi.org/project/vdf/