Is it dangerous if a phone used for crypto activity get stolen after formatted

[NotATether]

Yes, possible that they can recover files from the phone even if it is totally formatted. There is a tool called JTAG that mostly use for repairing phones and also use to recover files the process is a bit dangerous if you don't have knowledge of using JTAG it's direct to nand/memory IC and then can dump/recover your files instantly.

If you want to delete or erase them totally before you format your phone you need some apps called data eraser or shreddit to permanently delete files and then those files are irrecoverable. Now, you are safe to reformat your phone without worrying about sensitive files that you don't want to expose.

File recovery is not limited to SD cards, it can also work on hard disk drives and USBs (but not SSDs) that have a freshly cleared filesystem, though obviously a different tool is necessary to make use of that.

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[Falconer]

I can't remember how many old wallet files I've deleted from Phone and PC so far, but I can absolutely confirm that they won't be of any value anymore as no assets are stored on any of them. But fortunately so far I have never sold anything, be it my old Phone or notebook, it is still stored and has not been touched to this day.

After reading this one then I think one sure solution for me is to burn everything instead of selling it. Of course there are risks that we never think about selling them to a dealer even if they are formatted, but burning them is at least safe if it is no longer in use.

Yes, possible that they can recover files from the phone even if it is totally formatted. There is a tool called JTAG that mostly use for repairing phones and also use to recover files the process is a bit dangerous if you don't have knowledge of using JTAG it's direct to nand/memory IC and then can dump/recover your files instantly.

If you want to delete or erase them totally before you format your phone you need some apps called data eraser or shreddit to permanently delete files and then those files are irrecoverable. Now, you are safe to reformat your phone without worrying about sensitive files that you don't want to expose.

[erep]

I'm not sure formatted files can be recovered 100% from memory, ssd or other devices, I have tried to recover some deleted files in 1 week duration and only 30% of files can be recovered if we don't do regular cleaning on PC/mobile device. But if you doubt the file can be recovered then use a backup storage device to move important files, so you don't worry if you lose your phone or even you can replace the purchased phone to the latest version

[BitMaxz]

Since iOS 12 the trick of formatted data that wasn't written on it doesn't work anymore because of how APFS works, once the file is deleted the key is also, plus a long time ago iOS started encrypting the file system with a second key based on the first lock in user. So once you erase it with "erase Iphone" there is no way for someone to actually read the data even if recoverable, the only way to do it is from your backup.
That doesn't mean you should use your Iphone to store your keys either.

Files can be still recoverable for IOS12 but u3tools can clean/wipe your iPhone it uses Anti-recovery flash mode to clean and destroy personal data.
Time is fast technology keeps developing even if it is encrypted in the future hackers crackers can able to develop software that can decrypt recovered files from iPhones. So if you are going to give it to someone and don't want to expose your old important files that you recently deleted always use the Anti-recovery flash mode to fully destroy and clean your iPhone storage.

[o_e_l_e_o]

I can't remember how many old wallet files I've deleted from Phone and PC so far, but I can absolutely confirm that they won't be of any value anymore as no assets are stored on any of them.

But why? You can't be 100% certain you won't accidentally reuse an old address in the future, or someone who sent you coins to one of your old addresses won't send coins to that address again in the future, assuming you still have access to it. If it's a wallet on your phone or PC, as you say, then you have nothing to lose by keeping a copy just in case. The file will be few megabytes at most. In the words of Satoshi:

Sigh... why delete a wallet instead of moving it aside and keeping the old copy just in case?  You should never delete a wallet.

The other way around this issue is to use full disk encryption, which I use on all my devices. If you use a strong enough decryption key, then it doesn't matter if you leave some data behind after shredding, as it will be meaningless to anyone who is able to recover it.

[Falconer]

But why? You can't be 100% certain you won't accidentally reuse an old address in the future, or someone who sent you coins to one of your old addresses won't send coins to that address again in the future, assuming you still have access to it. If it's a wallet on your phone or PC, as you say, then you have nothing to lose by keeping a copy just in case. The file will be few megabytes at most. In the words of Satoshi:

Sigh... why delete a wallet instead of moving it aside and keeping the old copy just in case?  You should never delete a wallet.

o_e_l_e_o, thanks for reminding me of that post and yes I won't argue that it was good advice from then till now. What is clear I have saved some important files and also deleted quite a lot from the previous device that I can no longer use. However I don't plan on destroying the device in any way so far as my warehouse is still safe enough to hold more used stuff.

I also have 2 flashdisk currently to store important data. I check it regularly because I think there is a possibility about a device malfunction that could lose my data there. But over the last few years, it's still safe.