get_related_contexts_string() was deprecated by MDL-40468, and a debugging message added:

get_related_contexts_string() is deprecated, please use $context->get_parent_context_ids(true) instead.

This appears to be wrong, and should instead be suggesting calling the method with no parameter. The replacement code is calling get_parent_context_ids() with no parameter, which is equivalent to get_parent_context_ids(false), and this seems to tie in with the original functionality of the function which didn't return the context itself, only its parent contexts.

(The information is also repeated in /lib/upgrade.txt)