Will there be an iOS legacy client?

No, due to the fact that iOS is changing its ​push notification service​, which will only be compatible with v16.5. This means that even if you have a V15.5 PBX, for which you would need a ‘legacy’ app, it will not work anyway as the implementation inside the PBX will become incompatible with the apple service. 

On 1/6/2020 iOS clients will auto-update and will only function when the PBX is also on v16.5 or above.

APP updates are managed by the device itself. There is an option to disable all app updates on iOS which we strongly do not recommend to use and is only been added for transparency reasons. 

The Android legacy client will be deprecated on 1/7/20 when version 15.5 is no longer supported (EOL). Customers must upgrade to version 16 update 5. 

Version 16 was released 27th March 2019 ​(blog post​) which is over 14 months ago. As a partner charging/managing these customers maintenance/annual renewals, you are aware that your customers should be on the latest version, in order for them to have a fully secure and supported PBX

Each service pack includes numerous new features, bug fixes, and optimizations that overall improves the user’s experience with 3CX. Each version also increases its security which is changing all the time.

This is linked to Apple changing its push notification service. Their new service will only support HTTP/2. Because the PBX needs to communicate with this service, HTTP/2 must be used, and for this to be possible, the Operating System on which 3CX is installed needs to support it also.