It's not possible to answer a one-liner like
What is the latest on Mode S?
But if you want a one-liner...
For IFR you have to carry it. Done and dusted, more or less.
For VFR you don't have to, in some places, but the full story is complicated.
I'd say that the full story is so complicated that anybody doing any serious VFR abroad, VFR in CAS, or any IFR, may as well fit it rather than try to dig out the constantly varying exemptions list.