Currently the FetchXML in a subgrid can be amended using an unsupported way - but the FetchXML itself and also the ability to refresh a sub grid is supported - the ability to amend the FetchXML is massively useful as it allows all sorts of things otherwisae very difficult (for example viewing all activities in a single sub grid - using the Activity Party, or viewing "grandchildren" where there are two 1:n relationships (Entity A:B and then B:C - seeing all of C from A)
Thank you for your feedback. Currently this is not in our immediate roadmap, however, we are tracking it and if we indeed get more feedback and votes, we may consider it in future.
Thanks
Anusha Shankar