The WordPress Core will not use the legacy jQuery anymore. It will be upgraded from version 1.12.4 to 3.5.1. jQuery is a fast, small, and feature-rich JavaScript library.
https://www.yogeshchauhan.com/wordpress-is-migrating-to-latest-jquery-version-and-people-are-freaking-out/
https://www.yogeshchauhan.com/wordpress-is-migrating-to-latest-jquery-version-and-people-are-freaking-out/