Matrix in "slash" order

Jelly, 6 5 bytes

pSÞỤs

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How it works

pSÞỤs  Main link. Left argument: n. Right argument: k

p      Take the Cartesian product of [1, ..., n] and [1, ..., k], yielding
       [[1, 1], [1, 2], ..., [n, k-1], [n, k]].
 SÞ    Sort the pairs by their sums.
       Note that index sums are constant on antidiagonals.
   Ụ   Grade up, sorting the indices of the sorted array of pairs by their values.
    s  Split the result into chunks of length k.

Python 3, 91 bytes

def f(n,k):M=[(t//k+t%k,t)for t in range(n*k)];return zip(*k*[map([M,*sorted(M)].index,M)])

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R, 101 60 54 bytes

function(M,N)matrix(rank(outer(1:M,1:N,"+"),,"l"),M,N)

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Thanks to @nwellnhof for the suggestion of rank

Ports Dennis' Jelly answer.

Old answer, 101 bytes:

function(M,N)matrix(unsplit(lapply(split(1:(M*N),unlist(split(x,x))),rev),x<-outer(1:M,1:N,"+")),M,N)

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split is doing most of the work here; possibly there's a golfier algorithm but this definitely works.

Explanation:

function(M,N){
x <- outer(1:M,1:N,"+")			# create matrix with distinct indices for the antidiagonals
idx <- split(x,x)			# split into factor groups
items <- split(1:(M*N),unlist(idx))	# now split 1:(M*N) into factor groups using the groupings from idx
items <- lapply(items,rev)		# except that the factor groups are
					# $`2`:1, $`3`:2,3, (etc.) but we need
                                        # $`2`:1, $`3`:3,2, so we reverse each sublist
matrix(unsplit(items,x),M,N)		# now unsplit to rearrange the vector to the right order
					# and construct a matrix, returning the value
}

Try it online! -- you can use wrap a print around any of the right-hand sides of the assignments <- to see the intermediate results without changing the final outcome, as print returns its input.