Meta-issue for discussion and to keep track of all the individual changes required to achieve this goal.

It may be helpful to define three classes of tasks and an ordering as DE sees it

• First order: establish C++11 as the baseline allowing variadic templates (and the 'can use C++11' define) to reduce to unconditional code which, as a major side effect and first order goal, leads to removal of all the pre-C++11 autogenerated code. Other cleanups below inst/include and in src/ happen basically as a side effect
• Second order, by a wide margin: cleanup below inst/ and removal of old helpers or illustrations which does not really affect anyone
• Third order, and possibly tied with or marginally ahead of the previous point: cleanup of R helper functins

Tasks

• Clean up inst/include/Rcpp/platform/compiler.h. Ancient tests and defines that are not required anymore.
  "Spring cleaning" of some ancient pre-processor directives #1363
  Header cleanup with C++11 as a baseline #1364
• Review HAS_VARIADIC_TEMPLATES checks and remove carefully, also cleaning up inst/include/Rcpp/generated stuff. (DE: I expect this to be one single careful PR)
  Remove use of HAS_VARIADIC_TEMPLATES #1366
  Use variadic templates unconditionally #1367
• Review RCPP_USING_CXX11 checks and remove carefully. RInside uses this macro (see this search). Then, the definition could be removed. (DE: Thanks for the heads-up. I can take this.)
  Remove RCPP_USING_CXX11 which is now implicit #1369
• Find remaining if __cplusplus < 201103L and assess removal
  Cleanup __cplusplus checks based on C++11 availability #1370
  Cleanup __cplusplus checks #1371
• Review RCPP_USING_UNORDERED_* checks and remove carefully. Review RCPP_UNORDERED_* types and just replace with the appropriate std::unordered_* type.
  Remove macros for unordered maps/sets #1372
• Remove branch in inst/include/Rcpp/sugar/functions/sapply.h:41 (see TODO there). (DE: Unsure about this one, lesser priority for me. To be determined...). (IU: Maybe the other way around: maybe we should just remove the first branch, the one that uses the std library. Then ::Rcpp::traits::result_of could (or could not) be reimplemented based on the std library.)
  Enable lambdas in all sugar functions #1373
• Reach out packages jiebaR, fwsim, diveRsity (removed from CRAN), which use RCPP_UNORDERED_MAP and RCPP_UNORDERED_SET (see this search) in order to replace those uses too. We can deprecate these definitions and remove them in a future release.
• Maybe implement more variadic templates if any missing, then clean up remaining inst/include/Rcpp/generated stuff.
• Same for generated stuff under inst/include/Rcpp/module.
• Same for generated stuff under inst/include/Rcpp/sugar/block.
• Same for generated stuff under inst/include/Rcpp/sugar/functions/mapply.
• Clean up R/RcppLdpath.R. Functions that have been deprecated for ages. Some internal functions could just be dropped (also the inst/discovery script). The exported ones require some revdep checking. (DE: Sure but low priority as it has not bitten anyone. We could also chase packages still doing 'Depends: Rcpp' but ... to what payoff?)
• Clean up plugins in R/Attributes.R. Particularly, cpp98, cpp0x, probably cpp11 too. (DE: Unsure. There will be old code snippets hanging around that have this. Just as with old LdFlags() calls we could just make them null-ops)
• Update vignettes accordingly.

@eddelbuettel Feel free to add/drop/edit/reorder above.