LenientBoolean

A Boolean that also accepts the spellings YAML 1.1 counted as boolean.

Bukkit read config.yml as YAML 1.1, where yes, no, on and off are booleans; kaml reads YAML 1.2, where they are plain strings. Rejecting them would take a file that has worked for releases and quietly reset the setting - and for checkForUpdates: no the default is the opposite of what the file says, so the operator would get behaviour they had turned off.