Due to a recent criticism of R. T. Mullins by Ed Feser, people are discussing the modal collapse objection to divine simplicity. I thought I would make a few methodological points.
(1) Modal collapse is often created by imported assumptions. Modal collapses are not exclusive to matters of divine simplicity; they can arise in any modal context where an assumption or combination of assumptions relates possibility and necessity. One of the things that we learn from dangers of modal collapses elsewhere is that it is very easy to introduce an assumption yourself, distinct from the topic you are considering, that in itself creates the modal collapse. Thus, if you have modal collapse argument against anything, you need to be on your guard against the possibility that you might be creating it by introducing an assumption, whether explicit or implicit, that requires modal collapse.
This is definitely relevant to modal collapse objections to divine simplicity that make use of the possible worlds framework; over and over again, I have found such arguments to make the assumption that the actual world is one and only one possible world. As this assumption is nothing other than a formulation of unrestricted necessitarianism, using it in any argument on any subject will induce a modal collapse.
(2) Modal collapse requires intersubstitution. Modal collapse objections with respect to God essentially involve variations of a basic structure: we have a description of God in terms of necessity (e.g., an argument that God necessarily exists) and a description of God in terms of contingency (e.g., an argument that God is creator of the contingent universe). Obviously these are just distinct descriptions unless we introduce something that we take to require us to jumble the modalities. This is in fact the only reason that divine simplicity has to do with anything in these contexts -- people making modal collapse objections are trying to claim that it requires us to substitute necessity-style descriptions and contingency-style descriptions for each other (in particular, because they take it to be just a form of identity). A common weakness in modal collapse objections is failure actually to establish this. People often get to something like "Divine simplicity requires that everything in God be in some way the same" and then assume, falsely, that this is strong enough to force us into the intersubstitution on which any modal collapse depends.
(3) Modal collapse objections need to be checked by parity. One thing I've been seeing more and more of is gerrymandering in an attempt to make it so that we can't move from modal collapse in divine matters to modal collapse in other matters (e.g., with free will, an area in which it is notoriously easy to get modal collapses by making imprecise assumptions). This shows that people at least recognize that they have to exercise some care here, but as modal collapse is actually a structural issue in argument, the structural tools that are used to get an alleged modal collapse in one case need to be tested in other contexts before you consider questions of whether the divine case might be unique. To put it in other words, you need to check the objection in one domain for potential parity problems in other domains without introducing any special assumptions for the domain. After you have done so, you may have uncovered the sort of problem noted in (1) or another problem, or else you might need to argue, independently, for some fundamental disanalogy between the cases, which makes the parity-premises false in one case and not in another. A particularly significant issue for divine simplicity is that simplicity is historically a term that can be used relatively: that is, traditionally everything that exists requires some kind of unity, and any kind of unity that rules out some kind of composition may be considered a kind of simplicity, and people have argued that (e.g.) virtuous character is more simple than vicious character, that angels are more simple than human beings, etc. This increases the likelihood that any modal collapse for divine simplicity will introduce analogous problems all the way down the line in the more restricted and limited cases; one would have to argue that some feature or other of the restrictions and limitations blocked it, which I have never seen anyone do.
