Which iPhones can write NFC tags?
Writing always needs an application open in front of you, on every model without exception. There is no background writing on iOS, so the interesting question is not which models write but the fact that the application requirement never goes away.
An app, always
Reading a tag can happen without you opening anything, on the models that support it. Writing cannot, on any model.
An application has to be open, in front of you, and asking to write. The system then manages the interaction and the application supplies the content.
Take NFC Tools. On iOS it requires an iPhone 7 or later running iOS 15.6 or later, and the app must be launched manually. Background reading is available from the iPhone XS and XR onwards.
So the reading threshold and the writing requirement are different kinds of limit. One is about hardware generation, the other is about how the platform is designed.
Why there is no background writing
Reading a tag is passive from the tag's point of view and reversible from yours: nothing changes.
Writing changes a physical object permanently, and on some tags irreversibly. A platform that let a backgrounded application modify tags a user brushed past would be handing out a capability nobody could supervise.
Whether or not you agree with the trade off, it is a deliberate design rather than an omission, which means it is not something a future update is likely to relax.
What this means in practice
Every write on an iPhone is a deliberate act with the phone in your hand and an application on screen. That is fine for production and awkward for anything automatic.
A flow that expects a tag to be updated when a user passes it cannot be built on iOS. That is a structural limit rather than a bug to work around.
It also means an iPhone cannot silently alter someone else's tag, which is the reassuring half of the same fact.
Choosing an encoding device
For producing a batch, either platform works and Android is generally the more permissive: a wider range of record types, and access to the layer beneath NDEF where configuration lives.
For a small run, an iPhone is perfectly adequate and the constraint is only that you are holding it.
What matters more than the encoding device is the verification. Read every tag back afterwards, and read it on the other platform as well, since a tag written successfully can still be inert on a phone that did not write it.
Where a deployment will be tapped by both platforms, test the outcome on both before producing the quantity.
Frequently asked questions
Can an iPhone write a tag in the background?
No, on any model. Writing requires an application open and in front of you, which is a design decision rather than a hardware limit.
Can an iPhone write tags an Android phone can read?
Yes. The tag is standard, and what differs afterwards is which record types each platform acts on.
Is Android better for writing?
It is more permissive, with a wider range of record types and access to the layer beneath NDEF. For ordinary content an iPhone is adequate.
Do I need a special app to write?
You need one that offers writing, since not every reading tool does. Reading and writing are separate capabilities in software.