Hi Christophe,
Appologies for my outburst...it seems you are correct and it is because of those changes to the core_renderer.
I have just been doing some hunting to see if those changes were mentioned in the theme/upgrade.txt for 2.6, It was but only for Maintenance, and then not specific to block regions in themes as such.
However there is a setting and an associated comment in Moodle 2.6/themebase/config.php that was not as informative as it is in 2.7(see below) which is where I first came across it, and since it is not used in Bootstrapbase themes we Theme designers don't need to use it.
MOODLE_26_STABLE - comment in ../theme/base/config.php (blocks_for_regions).
https://github.com/moodle/moodle/blob/MOODLE_26_STABLE/theme/base/config.php#L178-L180
MOODLE_27_STABLE - comment in ../theme/base/config.php (blocks_for_regions).
https://github.com/moodle/moodle/blob/MOODLE_27_STABLE/theme/base/config.php#L179-L181
So that said. there appears to be is a distict possibility that the setting found in theme/base/config.php that relates to the blocks_for_region needs adding to Aardvark/config.php.
Cheers
Mary