The documentation about which Android versions will be unsupported when the cross-sign expires doesn't directly address v7.1.
This document says that Android 7.0 or earlier will not be supported: Shortening the Let's Encrypt Chain of Trust - Let's Encrypt
This document says that >= Android 7.1.1 will be supported: Certificate Compatibility - Let's Encrypt
These documents also seem to say that 7.1.1 and above will be supported:
Will Android 7.1 (not 7.1.1) be supported?
Support for Let’s Encrypts root was added in 7.1.1. Previous releases, including 7.1, won’t support Let’s Encrypt.
Thank you for the clarification!