Here are the recommended settings for MSFS 2020 and MSFS 2024 to disable the in-game ATC and the built-in traffic injection system.

MSFS 2024

Disable the Built-In ATC:

Options, General, Sound, Character Voices Slider – 0
Options, Assistance, Realism, AI Radio Communications (ATC) – Off
Options, Assistance, Realism, ATC Enforce Flight Plan – Off
Options, Accessibility, Subtitles, Subtitles – Off

Disable the Built-in Traffic Injection:

Options, General, Graphics, Parked Aircraft Quantity – Off
Options, General, Graphics, Aircraft Traffic Quantity – Off
Options, General, Online, Aircraft Traffic Type – Off
Options, General, Online, Air Traffic in Career – Off

VR Users:
Options, VR, VR Graphics, Parked Aircraft Quantity – Off
Options, VR, VR Graphics, Aircraft Traffic Quantity – Off

Reminder:
Disable/remove any other add-ons that inject traffic.

MSFS 2020

Disable the Built-In ATC:

Options, Assistance Options, Piloting, AI Radio Communications (ATC) – Off
Options, Assistance Options, User Experience, ATC UI Panel – Off
Options, Assistance Options, User Experience, ATC Enforce Flight Plan – Off
Options, Assistance Options, User Experience, ATC Voices – Off

Disable the Built-in Traffic Injection:

Options, General Options, Data, Live Real-World Air Traffic – Off
Options, General Options, Data, Multiplayer – Off
Options, General Options, Traffic, Aviation Traffic, Aircraft Traffic Type – Off
Options, General Options, Traffic, Airport Life, Ground Aircraft Density – Off