Magento 2.1.18 is the final 2.1.x release. After June 2019, Magento 2.1.x will no longer receive security patches, quality fixes, or documentation updates.
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2.1.x Release Information

Magento 2.1.0 Release Notes

Backward-incompatible changes

Overview of backward-incompatible changes between the 2.0 and 2.1 releases: Magento 2.1 Backward Incompatible Changes

Third-party license agreements

Updated

Jun 17th, 2019