Yes, Until your kid finds it, then factory resets it because she couldn't get TikTok installed.

In general, yes you can use an old phone as cold wallet. Nothing wrong with that. But is kinda useless. Too much paranoia with air gapping.

A cold wallet you use it ONLY to deposit and check the balance (watch-only), so that scenario with air-gap it doesn't make sense.

Is enough to store the seed in a safe place and use that old phone to restore the seed only in emergency case or for a deposit. It takes few moments to restore it, no big deal.

The thing of keeping an old phone as a hardware wallet is just giving the impression of more security, but in the end is just an impression. Is like wearing a fucking mask. It doesn't serve to protect you.

The best way to protect you, from yourself (your own mistakes) and from other type of attacks, is to use the 3 levels stash: cold, cache, spending. Compartmentalizing your funds for each destination.

For the complete cold storage bitcoin - basically the bitcoin you're planning on keeping long term - if I had the bitcoin on an old phone, I think this might be more secure:

That makes it kind of like the Seed Signer.

Of course, you need to do this only for spending bitcoin - signing transactions. For receiving, just have a watch only wallet to get the receive address.

For the bitcoin wallet that needs to be be more accessible, that has less in it, then I think NOT deleting the wallet is fine.

The idea behind a hardware wallet is that you're assured that there is no way for the device to be compromised. It can do one thing and one thing only: sign transactions. It cannot broadcast transactions, it cannot do anything but receive unsigned transactions, sign them and send the signed ones back. It cannot connect to any network.

This is why hardware like ledger with their online backup recovery aren't good. This is why these newer devices that connect over Bluetooth are no good. You want something that has a single channel, either wired or using QR codes and a camera, that way physical access is required and the attack surface is low.

In light of this, a phone has just too high of an attack surface. It has a mobile radio, it has bluetooth, it has WiFi, and you may not know this but even without a sik card the radio connects to towers. It probably doesn't send any data, but the part of the device that connects to these networks is a separate environment and can be backdoored by a sophisticated adversary and pull information from the memory without you knowing. That's the "government is after me" vulnerability, but beyond that, the simple fact that it can connect to networks and run other applications makes it unsuitable.

If you need to do this, make sure you do it with only a small to medium stash. Think of it less like a savings account and more like a checking account for withdrawing cash from an ATM. Your actual savings, the chunk you don't withdraw from except in emergencies or like once a year or something, should not be stored this way.

It depends. If you can afford an old phone, you can definitely afford a hardware wallet. So, go for HWW, It's better.

You could, but why? Remember that what you're doing with secure cold storage is buy time: time between when your seed container gets stolen, and funds get cleaned out. The more time this takes, the more time you have to move your funds to safety.

Re: the "old" part, old phones often have hardware/firmware weaknesses that are actively exploited. As of April last year, everything older than an iPhone 12 or Pixel 6 (and apparently every Samsung ever made) is basically instantly(-ish) crackable through hardware/firmware/os exploits, so keep in mind that you may have a lot less time to move your funds out with older phone models; also see #616858

I'd not store the seed on the phone as a production secure device. Maybe for tinkering on testnet it can be fun.

Sentinel watch only wallet for use on the online phone and Ashigaru privacy focused Bitcoin wallet on the offline also works for this.

I remember this video about using blue wallet for multisig on a few offline phones.