I personally prefer to be explicit about targets but I wanted to give a try to:

(rule (action (write-file file.ext "content")))

and it fails with:

File "dune", line 1, characters 0-47:
1 | (rule (action (write-file file.ext "content")))
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Error: fields targets, target are all missing (exactly one is needed)

Is the rule target inferrence still working?
It seems like it's not, I just tried the example from https://dune.readthedocs.io/en/latest/dune-files.html#inferred-rules and I'm getting the same error.

I'm totally fine with this feature being removed but we should probably delete the doc section advertising it then!