I’m curious why you say that Kant employed a Newtonian conception of space/time as pure forms of intuition? Rather it seems that Kant would say that the pure forms of intuition make a conception like Newton’s or Einstein’s possible. I say this because pure intuition is that through which we can construct concepts a priori, and it is only due to this capacity that the objects of mathematical physics are taken to have an a priori foundation (see Preface of Metaphysical Foundations of Natural Science).

I have never been satisfied by readings of Kant, such as Russell’s, that seem to overlook the fact that the pure forms of intuition are not grounded in work carried out by any particular mathematical discipline. Russell believed that space in Kant was essentially Euclidean, where Kant would say rather that Euclid’s Geometry has a foundation in our capacity to generate concepts a priori using pure intuition. Claiming that the pure forms of intuition are Newtonian is making a similar error in understanding the priority of pure intuition as Russell made.

I agree that mathematical thinking has advanced, and has been able to place physics on new foundations. However, the manner in which this has been carried out is entirely in agreement with Kant’s understanding of natural science that we find in the Metaphysical Foundations of Natural Science. Even though Kant’s own grounding of the concept of ‘matter’ in that book was restricted to the mathematics which Kant employed, the heart of this isn’t the particular working out at any time of the basic concepts of a science, but 1) the requirement of mathematics for founding natural science on an a priori basis, and 2) the a priori status of mathematics depending on our capacity to generate concepts of a spacial/temporal nature without first finding them in objects of experience (that is, pure intuition).