Safari for iPad and iPhone provide “AutoFill” integration from third-party password managers (e.g. 1Password, Microsoft Authenticator, Chrome and Lockwise (Firefox) etc). This capability is available in iOS12 or later versions of iOS/iPadOS.
Older versions of iOS and Safari do not offer this integration with Safari. Whilst older devices do not benefit from automatic AutoFill of credentials from third-party Apps, subject to meeting OS requirements, Apps themselves may still be usable. The Apple App Store indicates that 1Password requires iOS12.2 or later - and as such limits the devices capable of using 1Password. Other third-party password manager Apps may be similarly constrained.
Whilst you may not be able to use some Apps with older devices (most of which are technically obsolete - and may not technically “secure”), you can still use 1Password and similar Apps with newer devices without issue.
I hope this clarification proves to be helpful to you.