can i still downgrade to ios18

Not only is it not safe, it will not work. Users who have tried found that iCloud backups no longer work along with other iCloud syncing issues. There is a reason why Apple has never supported downgrades, and actually the same is true for Android where some have found their devices bricked when attempting the same thing.

NEVER download software from a third party site and install it on your phone. That is similar to a Jailbreak that eliminates the security on your device, leaving or personal data and security at risk.

That's not how signing works. There is a digital signature in the file that is used to verify it is authentic. The iPhone will examine the file and contact Apple servers to determine if the digital signature is still valid. Apple has marked that signature as no longer valid. The iPhone will refuse to install it.

teokpn7 wrote:

Apple signs the ios files offered by the ispw.me site itself, for example as it signed ios18 2 weeks ago, if there was an unsigned version i would not have downloaded it but as i explained above ,i’m confused because i downloaded it when it was signed before. also i’m using itunes to downgrade device

You may have downloaded it while it was still being signed. However, that's no longer true. That means, should you try to use it, it will fail.

And that site is not an official Apple site.

teokpn7 wrote:

Apple signs the ios files offered by the ispw.me site itself, for example as it signed ios18 2 weeks ago, if there was an unsigned version i would not have downloaded it but as i explained above ,i’m confused because i downloaded it when it was signed before. also i’m using itunes to downgrade device

Not usable and not provided by Apple. Do you really think Apple is sending the ipsw to that site? The answer is NO. It is a mistake to think that being signed by Apple means that this is an authorized method to downgrade. It is not. Just because you can jailbreak a device, does not mean you should.

teokpn7 wrote:

i downloaded ios18 from ispw.me when it was signed (to my pc) and apple stoped signing ios18, can i still use it? is it safe

It's your phone. Do what you want with it. But you asked us, many of whom have years of experience with topics like this and the answer is, you may render your phone unusable and Apple won't help you, nor can anyone here help you. It just can't be worth this to downgrade.

If you have downloaded iOS 18 from any other place then software update on your iPhone or the official way to update your iPhone via a pc or Mac using the finder or iTunes I would directly upgrade from software update to make your device safe again