It is per target, that's how it works when I've used it. For example:

  # Makefile
  SHELL := bash
  .ONESHELL:
  .RECIPEPREFIX = >
  
  thing1:
  > FOO=bar
  > echo "$${FOO:?}"
  .PHONY: thing1
  
  thing2:
  > echo "$${FOO:?}"
  .PHONY: thing2

Results in:

  $ make thing1 thing2
  FOO=bar
  echo "${FOO:?}"
  bar
  echo "${FOO:?}"
  bash: line 1: FOO: parameter null or not set

Note how the bash error for unset FOO is line 1 for the second target.

Edit: Maybe I misinterpreted, do you mean you'd want to choose whether a given target is ONESHELL or not?