The value of the variable "table_definition_cache" is by default autosized (based on the table_open_cache value) on MySQL >=5.6.8 while it is still static (400) on the latest MariaDB 10.0.14 and 10.1.?, is that by choice or this feature hasn't been backported and should be?

From the MySQL doc http://dev.mysql.com/doc/refman/5.6/en/server-system-variables.html#sysvar_table_definition_cache :

Permitted Values (>= 5.6.8)
Type	numeric
Default	-1 (autosized)
Min Value	400
Max Value	524288

The default value is based on the following formula, capped to a limit of 2000:

400 + (table_open_cache / 2)

Before MySQL 5.6.8, the default is 400.