There are 5 remaining mentions of Execution Character Set in the draft:

[character.seq.general] (1.2)
The execution character set and the execution wide-character set are supersets of the basic literal character set ([lex.charset]). The encodings of the execution character sets and the sets of additional elements (if any) are locale-specific.
[Note 1: The encodings of the execution character sets can be unrelated to any literal encoding. — end note]

3.35[defns.multibyte]
multibyte character
sequence of one or more bytes representing the code unit sequence for an encoded character of the execution character set

[lex.charset] 6
A literal encoding or a locale-specific encoding of one of the execution character sets ([character.seq]) encodes each element of the basic literal character set as a single code unit with non-negative value, distinct from the code unit for any other such element.

Is it the set of all possible characters in any encoding that is supported? Because that seems an awfully broad set.

While it's probably worthwhile not to break someone's existing C++ reference material, I'm not sure we have a crisp and clean definition here, nor am I sure that multibyte character should be tied to it? 

I don't have any concrete suggestions here, but I was trying to help someone else understand the new model, and they had questions that were harder to answer than I expected.