UserSettings is not available anymore since 2.14.0-b1, but is not removed since Update file requires it to be installed prior to beeing removed. And this plugin don't require installation so it never land in
PluginsInstalled section. In effect plugin remains in config file when updating to 2.14.3 and throw an error.
Here is proposed solution: https://github.com/piwik/piwik/pull/8658
#8658 was closed, we didn't think there was a bug. if there is, please let us know how to reproduce, and re-open. cheers