You'll see in a parallel message that I've just sent an expl3 release to
CTAN. There are two important changes to highlight, both deprecations.
When expl3 was first written, the only required primitives were from
TeX90, and all could happily be named \tex_...:D. e-TeX primitives, when
added, were named \etex_...:D to keep things 'clear'. Subsequently,
pdfTeX, XeTeX, LuaTeX and (u)pTeX have all added primitives, and some
have been ported between engines. To date, these have been stored under
engine names, e.g. \pdftex_...:D, but that is increasingly complex.
The team have decided to move *all* primitives to the \tex_...:D
namespace, and to deprecate the older names. As direct primitive access
should only be used where there is no expl3 interface, and should take
the form of
\cs_new_eq:NN \__pkg_local_name:w \tex_...:D
the number of public packages impacted is small.
The standard deprecation period (running to the end of 2019) applies to
the older names.
The naming of integer constants has to-date been non-standard: \c_one,
etc. At the same time, the use of constants is less important than it
was when expl3 was first developed: the routine application of integer
expression processing means that termination of numbers is now not an
issue, whilst performance of modern PCs means that the time saving
compared to explicit values is *extremely* small.
We have now deprecated most of the constants, and moved those retained
to the naming \c_<value>_int.