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Polynomial rings have monomial orderings (in this case it is degrevlex by default), and the terms of polynomials are automatically ordered from greatest to smallest (with respect to the monomial ordering), as you can see e.g. with print(f).

You can get the exponents you want like this:

sage: [e for e in f.exponents() if all(e_i % 5 == 0 for e_i in e)]
[(10, 5, 0)]

Or the indices of the respective terms (again, with respect to the monomial ordering):

sage: idc = [k for (k,e) in enumerate(f.exponents()) if all(e_i % 5 == 0 for e_i in e)]; idc
[0]

Or the terms themselves:

sage: sum(prod(t) for (k,t) in enumerate(list(f)) if k in idc)
x^10*y^5

A bit more efficiently:

sage: terms = list(f)
sage: sum(prod(terms[k]) for k in idc)
x^10*y^5