For their intended purpose — quick signups, one-off downloads, and testing — disposable email addresses and temporary phone numbers are safe and widely used. Because the identity is not connected to your real accounts, a leak at a careless service exposes nothing about you. Your personal inbox stays off spam lists, and the temporary identity simply expires after use, taking every promotional email with it.
The important rule is to match the tool to the situation. A temporary inbox is a public inbox: anyone who knows the address can read its messages. That makes it the wrong choice for banking, healthcare, government services, work accounts, or anything you expect to keep long-term, since password resets and account recovery links arrive by email — and once the inbox is gone, so is your way back in. Use a durable, private address for accounts that matter, and a disposable one for everything you want to try without commitment.
Verification codes deserve the same care. Reading an OTP in a temporary inbox is perfectly fine for testing a new app, but never use a shared public number to secure a real account: the next person to receive a code on that number could be someone else entirely. If a service sends sensitive codes, secure it with a number or address only you control.
In short: for protecting your privacy during everyday signups and QA testing, temporary identities are one of the simplest safety tools available — just reserve your real contact details for the accounts you cannot afford to lose.