Since a brand new theme is coming in Moodle 3.2 core which will be based on Bootstrap 4 framework, Damyon Wiese – has proposed to deprecate the non Bootstrap themes – Base and Canvas Themes from Moodle core and move them to the plugins directory.
The main reason as cited by Damyon behind this proposal is:
They are not tested / maintained well and including them in core sends the wrong message. They are no longer professional quality themes that someone should be using as a base for a new theme.
Other Moodle theme designers like Mary Evans, Richard Oelmann, Dan Poltawski have also supported this proposal. Both themes will be first moved to the Plugins directory will allow organisations, that need those themes for their sites to work, to install them as ADD-ONS in the Plugins installer.