Extbase reflection cache is not configurable in ext_localconf.php
Since commit https://review.typo3.org/#/c/54381/ (released on in v9) it is no longer possible to configure 'extbase_reflection' cache in ext_localconf.php files.
The cache is instantiated early during ext_localconf.php by EXT:extbase.
Object\Container is configured (instanciated) in EXT:extbase/ext_localconf.php.
Object\Container then instanciates the
ReflectionService in its constructor,
which itself creates the extbase_reflection using the CacheManager
(all of that during ext_localconf.php loading).
After CacheManager has created the extbase_reflection cache once, it will never be recreated again,
when the final configuration is set, after all ext_localconf.php files have been loaded.