Couple banks not supported the version 12.5.4, iPhone 6 have this is the latest version .

Your banks decided to not support older versions of iOS, not Apple.

You have old hardware that doesn’t have enough RAM to support newer versions of iOS.

Apple supports iPhones with newer versions of iOS for 5 years historically.

Android phones get maybe 2 years.

Buy a newer phone that supports newer versions of iOS.

Buy a phone that supports a newer version of iOS

They are using an app from the bank, not a browser, and the bank has decided to update the app and not support anything pre-iOS13. That is up to the bank. They already told the OP where they stand. This is not at all unusual due to security issues.

Your comment regarding banks, schools and web sites not supporting Safari is totally incorrect and should not be posted here. But since the issue is with an app it is irrelevant in this case as is your information regarding clearing files.

kommaramesh Said:

"Couple banks not supported the version 12.5.4, iPhone 6 have this is the latest version .how I can fix this issue to use my apps.."

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Three Things to Consider:

A. Contact Your Bank:

You bank likely has yet to do its part in the compatibility. So, contact them, asking them where this all stands.

B. Don't Use Safari:

Banks, Schools, and Work server Web sites typically so not support Safari. So, use Google Chrome, or Mozilla Firefox, instead. Get them both on the App Store.

C. Clear Temporary Internet Files:

Clear your Temporary Internet Files(Cookies & Cache) of Safari:

• Cookies perform remembrance of certain site-entered data.
• Cache keeps a snapshot of the last time you visited a site.
• Go Here: Clear the History and Cookies from Safari on your iPhone, iPad, or iPod Touch - Apple Support

deggie Said:

"[...]Your comment regarding banks, schools and web sites not supporting Safari is totally incorrect and should not be posted here. But since the issue is with an app it is irrelevant in this case as is your information regarding clearing files."

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Why Banks, School, & Web Sites:

Make use of those places on your own time, and you just might run it by them to find out what's going on. It is true where I live, and the sites I use. Find out through the Web Master and you might just find out that incompatible can be an issue.

Maybe 10 years ago this was a problem, but with each iteration of Safari, and the growth in the use of it the problem has mostly gone away. If the place you live is woefully behind the times then to make your statement accurate you need to say, "In Transylvania where I live..

But it is certainly not true in most places anymore. Are there still sites that have some issues with it? Sure, but there are sites that stayed with Flash to the bitter end.

deggie Said:

"[...]But it is certainly not true in most places anymore. Are there still sites that have some issues with it? Sure, but there are sites that stayed with Flash to the bitter end."

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For Me:

For privacy purposes, I'm not going to provide those sites - but they just don't let their sites works with Safari. It's beyond my call.

All the best.

Privacy reasons? So these are private banks and schools not available to the public? And how do they make their sites not work with Safari? And how many thousands of them are there?