Do delivery apps have to warn me before they change their terms?

It depends where you are. In the EU platforms must give at least 15 days' notice of changes. In New York City a fee change cannot start until 30 days after notice. Zomato's terms say changes apply as soon as they are posted.

• EU: at least 15 days' notice of terms changes, and 30 days' notice with reasons before ending a contract.
• New York City: a fee change takes effect no earlier than 30 days after notice.
• Zomato: its terms say it may change them effective as soon as they are posted.
• Check: read the notice clause in your own contract.