Machine learning, deep learning and reinforcement learning are all on-topic here, but we ask that questions not be primarily concerned with programming.
I'm not active on DSCI.SE or AI.SE so I have no deep understanding of what is on-topic on those fora. However, I have been active on stats.SE for several years and I regularly participate ...
I think that this kind of issue is endemic to SO and will only increase as sites proliferate.
A proposal: there should be a proposal process where users can vote to merge sites, resulting in a permanent redirect.
Really, what's in a name? Cross Validation is an excellent name to catch the attention of anyone who's actually exercised curiosity + healthy ...
As a mental model, it can be helpful to consider the scope of Stats.SE and AI.SE as two circles of a Venn diagram.
Some topics, like sigma-algebra or statistical hypothesis tests, are only on-topic on Stats.SE.
Some topics, like machine-learning and reinforcement-learning, are on-topic on both sites.
Some topics, like philosophical or social consequences of ...