There are a lot of great thoughts here, and some impressive analyses (you know who you are), but I just wanted to pick up on this:
There is a constant battle going on from generation to generation over the soul of our society, and this is certainly reflected in what is considered acceptable humour - it feels almost Darwinian, the way this is played out. But I feel that picking up on the humour is really only dealing with the symptoms of underlying, competing social norms rather than connecting with the heart of these. It is certainly possible to use it as a diagnostic, though any one individual or group could well be unrepresentative.
I think you're right here, John, and what's more I think that it's a natural human instinct to behave in such ways ingrained on the level of Darwinian group selection and even scales of selection above that.
Ingroups and outgroups are constantly forming and reforming driven by selective pressures which at bottom have evolved to ensure the general fitness of the group as a whole. These eusocial mechanisms of group selection determine who is 'fit' and 'capable' enough to add to the capacity of the group's survival, and on the contrary who is 'weak' enough that group fitness might be harmed by their presence. In this case, humour is used as a tool to gauge intelligence as well as the skills required to promote social cohesion, much like its role in individual sexual selection.
What I find fascinating is how this interacts with other mechanisms of social network polarisation, and particular the almost automatic process whereby 'unitary utopias' are split into 'bipolar states' (these are the only two 'balanced' states of social networks, after the work of Fritz Heider and Harary & Cartwright). Almost every idea/opinion/&c. is drawn into the process of polarisation through oppositional mechanisms of value attribution, even the most seemingly irrelevant things, and the process seems to be, on the whole, generally irreversible. It might even be (probably is) another selective mechanism: split, conflict and conquer; split, conflict and conquer, cycling through unitary state > bipolar states > creative destruction > unitary state. In such ways does the group apply selective pressure to itself and increases its general fitness within the overall environment. There aren't many social or eusocial species who
don't do this, and of course those that wouldn't would eventually perish to external (extra-species) threats.
The interesting interplay here is that there's a conflict between the lower-level individual and group selective uses of humour and the general state of social-network polarisation today in the West, with this question of 'offensiveness' being held in polar tension between Right and Left. Ten or twenty years ago, 'poltical correctness' was more firmly a feature of the Left, and political incorrectness of the Right (though its been changing hands regularly for God knows how long - it's not a new concept by any means), though it's not as clear today.
The problem (and the conflict between selective mechanisms at different levels of the social structure) is that once you're polarised into the side that's currently associated with 'political correctness', then your lower-level individual/sexual and group selective fitness will start to be harmed. There's a tension there which will probably only be resolved with the marginalisation and elimination of the 'sub-clique' of SJWs from the Left more generally (since tripolar states are 'unbalanced' and hence unstable) - their prominence in the media isn't a sign of their cultural triumph or ascendancy, but quite the opposite. It's a sign of their differentiation from the more general 'Left' clique under which they previously subsisted, and the fact that the mechanisms of bipolarisation are kicking them out of the bipolar structure as a whole. 'SJW's as a phenomenon won't survive the next fifteen years as a serious political force. After that, the situation will revert to the Right being associated with being offended more generally, and the Left resuming its 'irreverence' (until it flips again).