The MSI installer for Jenkins 2.562 and later is signed with a new organization, LF Open Source, LLC and a new signing key. There are known issues with the user experience because Microsoft Defender Smartscreen intentionally presents a warning dialog for unfamiliar installers, even when they are signed with valid certificates.
Jenkins has been open source since it was created. Yes, Jenkins is still open source and will continue to be open source.
We’re deeply grateful for organizations that contribute to Jenkins and we’re especially grateful to CloudBees and the customers of CloudBees that have sponsored significant Jenkins development for many years.