Birch doesn't perform inplace operations (at least not on the input array), so the copy parameter is useless and should be deprecated. It's even detrimental because by default it makes a copy.

The only place where an inplace operation happens is in the update method of _CFSubcluster:

However, update is call in 2 places. The first one is in the _split_node function, but here we first create 2 new _CFSubcluster objects and so the update performs inplace operations on newly created data, so the input data is not modified. The second one is in the insert_cf_subcluster method of _CFNode but is only triggered if the subcluster has a child, which can only come from splitted subclusters (i.e. after _split_node), so again we're not modifying the input data.