Hey, thanks for the detailed FR.
It looks like the main use case a clean index without losing the ability to explicitly share a file is already covered by a separate file .cursorindexingignore. It only affects indexing, but it does not block reading via @ mention or drag and drop. So if you want an index-only ignore, you should use it instead of .cursorignore.
As for .cursorignore, the current behavior is intentional. It is a security boundary, files listed there should not reach the model through any path index, context, or attachment. So an override like include anyway is unlikely, because it breaks that guarantee.
But the idea of showing a warning instead of silently discarding is reasonable. Right now it is not obvious why drag and drop does nothing. I will keep the thread open and see what the community votes for.
Related: How to force Cursor to index files/folders in gitignore How to force Cursor to index files/folders in gitignore, it discusses the difference between .cursorignore and .cursorindexingignore.