While there is some confusion at work in how the question is asked, I think there's a good question in there somewhere. I first want to suggest the following opening version of the question: What is similar in Plato and Kant's respective accounts of where the truth is found?
I take on a basic level their main point of agreement is that truth is not out there in the world. Thus, for Kant, our apparatuses -- the categories of the understanding and the forms of sensibility are more important things to look at than things to discover truth. For Plato, truth is the Forms/Ideas. Again, these are not out there in the world. Instead, what we have in the world are Demiurge's poorly made copies.
With this rough sketch in mind, we can then think about what is different in the two views. In the Kantian view, the things themselves are there but we cannot access them as such. We turn them into objects and perceptions as we use our minds on them. So the categories apply concepts and the forms of sensibility render them in space/time as things we can perceive. N.b., I am using Kant's language here so if it sounds crazy don't blame me! So the important bits that make knowledge possible are in our heads.
In the Platonic account, what we have in our head are forms. What we see in the world are the shoddy copies that are, per the Cave, like shadows. Depending on where we are reading in Plato, the degree to which we remember the forms/know the forms differs. So in Apology and Euthyphro, we are unable to really get to knowledge. We're stuck with our inadequacy. In Meno and several later dialogues, we know the forms in our souls (use minds if you prefer) and just need to jog our memories. In a weird way, that means for Plato we are already knowers of what is most real.
This finally enables us to look at the question as you worded it:
did Plato ever speak about this impossibility of knowing things "as themselves"? ... whether Plato had any views about the impossibility of knowing the 'noumenon'.
Part of your question is a misinterpretation of Kant. noumenon are not things in themselves, but we can address each piece separately. The misunderstanding happens because of a confusion about how knowledge works Kant. While we cannot know the things themselves, this is because of how we know. [God, in fact, can know them.] because God does not filter things through forms and categories.
The answer is that Plato does think we can know what Kant might call "noumenon" but that's because the closest thing is the forms which is in soul. On the other hand, the things behind the phenomenon are on Plato's view either the forms or just the shifting shadows of the world we live in which is inadequate to contain forms.
What the two have in common then is a belief that the real goods are in the soul from before birth / in the mind of a rational being as categories and that the world is shadow. The neo-Kantians of the 19th Century were also Platonists after a sort, so you're not the first to note the similarity.