Find the option labeled "Sync Contacts" and toggle the switch OFF. It will typically turn gray or left-aligned to indicate it's disabled.
How to Delete Already Synced Contacts from Telegram's Cloud:
Disabling "Sync Contacts" only prevents future uploads. To remove contacts that were previously synced, you need to take an additional step:
Follow steps 1-4 above to reach the "Contacts" section within "Privacy and Security."
Look for the option labeled "Delete Synced telegram number database Contacts" and tap on it.
What Happens After You Stop Contact Syncing and Delete Synced Contacts?
No New Automatic Additions: Telegram will no longer automatically upload new contacts from your device's address book to its cloud servers.
Your Existing Telegram Contacts Remain: Any contacts you've manually added to Telegram, or contacts you've chatted with previously, will still appear in your Telegram contacts list and chats. Deleting synced contacts only removes those that Telegram identified and added from your phone's address book.
Manual Contact Addition: You can still add new contacts to Telegram manually. You'll need their Telegram username or their phone number, and you can add them directly within the Telegram app.
"Contact Joined Telegram" Notifications: If you've also configured your phone number privacy settings appropriately (e.g., "Who can find me by my number?" set to "Nobody"), you should no longer receive notifications when people from your phone's address book (who you haven't manually added or chatted with) join Telegram. Similarly, they won't automatically know you're on Telegram just because they have your number (though if they have their contacts synced, and have your number, they might still be notified when you join).
Your Phone's Address Book is Unaffected: This action only affects Telegram's copy of your contacts; your phone's native address book remains untouched.
By performing both actions (disabling sync and deleting previously synced contacts), you gain a higher degree of control over your privacy on Telegram regarding your phone's contact list.