Jenkins core currently uses Prototype 1.7, released on November 15, 2010. The latest version is Prototype 1.7.3, released on September 22, 2015. When an attempt was made to upgrade to 1.7.3 in 2018 in JENKINS-49319, the change had to be reverted. Since this library has been unmaintained for the past 8 years, we ought to eliminate our dependency on it in favor of modern JavaScript APIs. Once core no longer depends on this API, it should be investigated how this old library can be disabled/removed by default, enabled/present only for the (legacy) plugins that still require it.