I never understood why people would object to Kant’s thesis of “things in themselves”, or find it inconsistent with his epistemological scruples. I take this just to mean that there are ways that things are. This is an entirely separate question from whether we have perfect or certain knowledge of those ways. All that is ruled out by Kant’s Critical perspective is claims that we have knowledge of things just as they are in themselves. This just calls for a kind of epistemic modesty. (See also Kantian Discipline; Copernican; Dogmatism and Strife; Transcendental?)
People who rejected things in themselves included Fichte and the important early 20th century English translator and interpreter of Kant, Norman Kemp Smith, who was sympathetic to the phenomenalism then fashionable among empiricists (see brief discussion under Empiricism).
Hegel too was very critical of the phrase “things in themselves”, mainly because he thought the wording implied a kind of artificial isolation, but he by no means wanted to throw out the realist moment that Kant always wanted to affirm — quite the opposite. Discussions about realism and idealism get rather complicated, especially where Kant and Hegel are concerned, but Kant repeatedly affirmed a kind of empirical realism. I take this to have been a sort of pragmatic vindication of common sense with respect to ordinary experience, coupled with respect for Newtonian science. What Kant and Hegel both objected to — each in their own different terms — were strong traditional metaphysical claims. Whatever their other many differences, commentators are basically unanimous in taking Hegel to have wanted to be at least as “realist” as Kant.
Leibniz had suggested that God’s single eternal act is the selection of the best of all possible worlds from all possible worlds, and that in this context the complete essence of a thing is foreseen by God, allowing for a version of particular providence. Even though he worked out an alternative scheme for more concretely relational determination of essences in late correspondence with the Jesuit theologian Bartholomew Des Bosses, Leibniz preferred to stress the predetermination of each individual monad by God, as part of a comprehensive pre-existing harmony that put the reality of relations in the mind of God rather than in the world. In this context, Leibniz also famously suggested that monads do not really interact and “have no windows”. For Leibniz, “windows” to the outside are not really needed, because each monad contains within itself a reflection of the entire universe.
One of Kant’s earliest moves, however — long before publication of the First Critique — was to argue against Leibniz for common-sense real interaction among things in the world. It is doubtful that Kant knew of Leibniz’s alternative scheme for real relations, but in any case, Kant throughout his career stressed immanent relations among things in the world, rather than transcendent relations realized primarily in the mind of God. Since even the pre-Critical Kant had thus already undone the basis for treating each monad in splendid isolation, it seems very unlikely that the Critical Kant meant to imply any strong monadic properties when he spoke of things “in themselves”.
Saying that there are ways that things are does not have to mean that everything is determined down to the last detail. (See Equivocal Determination.)