Hello all,

The recent discussion about expl3 updates breaking unicode-math has 
prompted the team to look again engine-variation in expl3. In 
particular, the approach of using category code tables for 
\ExplSyntax..., while attractive, leads to problems as the behaviour of 
different engines is too different. Thus this idea is being removed from 
expl3, with the standard method being used to set up the code 
environment irrespective of the engine in use.

This does not mean that support for category code tables themselves is 
dropped. When used with LuaTeX, expl3 (as a package or in format mode) 
provides a mechanism for managing category code tables and associated 
macros. For the expl3 package, this is done by loading Heiko Oberdiek's 
luatex package, whereas in format mode the same ideas are implemented 
directly.

There will be a CTAN update of expl3 soon (by the weekend, I hope). This 
will include these changes (along with others: see my other e-mails).
-- 
Joseph Wright