SonarQube v5.6 LTS went EOL in May 2018 (6 months after the v6.7 LTS was released), so you can trust that SonarQube v5.1.1 went EOL long before that (and long before now).
All non-LTS versions of SonarQube are “technically” EOL (meaning there will be no more fixes released) once the next version is released. This isn’t super formalised, but theoretically an EOL date could be considered November 2015, with the release of SonarQube v5.2