Patch 2016 xparse to support k-type argument

With the current xparse release (2017/12/16), the following will re-add the k-type argument:

\documentclass{article}
\usepackage{xparse}
\usepackage{xcolor}

\ExplSyntaxOn

\tl_if_exist:NF \c_novalue_tl
  { \tl_const:Nx \c_novalue_tl { \c__xparse_no_value_tl } }
\cs_if_exist:NF \__xparse_single_char_check:n
  { \cs_set_eq:NN \__xparse_single_char_check:n \__xparse_single_token_check:n }
\cs_if_exist:NF \__xparse_add_type_K:w
  {
    \cs_set_protected:Npn \__xparse_tmp:w #1
      {
        \cs_new_protected:Npn \__xparse_normalize_type_k:w ##1
          { \__xparse_normalize_type_K:w ##1 {#1} }
      }
    \exp_args:No \__xparse_tmp:w { \c_novalue_tl }
    \cs_new_protected:Npn \__xparse_normalize_type_K:w #1#2
      {
        \__xparse_single_char_check:n {#1}
        \quark_if_recursion_tail_stop_do:nn {#2} { \__xparse_bad_arg_spec:wn }
        \tl_put_right:Nn \l__xparse_arg_spec_tl { K #1 {#2} }
        \tl_put_right:Nn \l__xparse_last_delimiters_tl {#1}
        \bool_set_false:N \l__xparse_grab_expandably_bool
        \__xparse_normalize_arg_spec_loop:n
      }
\cs_new_protected:Npx \__xparse_add_type_K:w #1#2
  {
    \exp_not:N \__xparse_flush_m_args:
    \exp_not:N \__xparse_add_default:n {#2}
    \cs_if_exist:NTF \__xparse_add_grabber:N
      { \exp_not:N \__xparse_add_grabber:N }
      { \exp_not:N \__xparse_add_grabber_optional:N }
        K
    \tl_put_right:Nn \exp_not:N \l__xparse_signature_tl { #1 }
    \exp_not:N \__xparse_prepare_signature:N
  }
    \cs_new_protected:Npn \__xparse_grab_K:w #1#2 \__xparse_run_code:
      {
        \__xparse_grab_K:NnNn #1 {#2}
          \cs_set_protected_nopar:Npn
          { _ignore_spaces }
      }
    \cs_new_protected:Npn \__xparse_grab_K_long:w #1#2 \__xparse_run_code:
      {
        \__xparse_grab_K:NnNn #1 {#2}
          \cs_set_protected:Npn
          { _ignore_spaces }
      }
    \cs_new_protected:Npn \__xparse_grab_K_trailing:w #1#2 \__xparse_run_code:
      {
        \__xparse_grab_K:NnNn #1 {#2}
          \cs_set_protected_nopar:Npn
          { }
      }
    \cs_new_protected:Npn \__xparse_grab_K_long_trailing:w #1#2 \__xparse_run_code:
      {
        \__xparse_grab_K:NnNn #1 {#2}
          \cs_set_protected:Npn
          { }
      }
    \cs_new_protected:Npn \__xparse_grab_K:NnNn #1#2#3#4
      {
        \tl_set:Nn \l__xparse_signature_tl {#2}
        \exp_after:wN #3 \l__xparse_fn_tl ##1
          {
            \__xparse_add_arg:n {##1}
          }
        \use:c { peek_meaning_remove #4 :NTF } #1
          { \l__xparse_fn_tl }
          {
            \__xparse_add_arg:o \c_novalue_tl
          }
      }
  }
\ExplSyntaxOff
%
\NewDocumentCommand\MyMacro{k_}{%
  \IfNoValueTF{#1}{%
      \mathbf{A}%
  }{%
      A_{\textcolor{red}{#1}}%
  }%
}


\begin{document}

With a subscript: $\MyMacro_{\pi}$ 

Without any subscript: $\MyMacro$ 

\end{document}

---

I have raised the transition question with the team, and will either update the answer here on the next release (if required) or arrange for k-type to have some (limited) transitional support. Note though that this entire area is still very much in flux!

For the simple case where it is used with a single token e and k are more or less the same so if you just make \IfNoValue a little less fussy abut the difference between {-NoValue-} and -NoValue- then you can alias k to e if it's not already defined so your MWE drops down to:

\documentclass{article}
\usepackage{xparse}
\usepackage{xcolor}

\makeatletter
\ExplSyntaxOn
\ifx\__xparse_normalize_type_k:w\undefined
  \def\__xparse_normalize_type_k:w{\__xparse_normalize_type_e:w}
  \let\xIfNoValueTF\IfNoValueTF
  \def\IfNoValueTF#1{\expandafter\xIfNoValueTF\expandafter{\@car#1.\@nil}}
\fi
\ExplSyntaxOff


    \NewDocumentCommand\MyMacro{k_}{%
        \IfNoValueTF{#1}{%
            \mathbf{A}%
        }{%
            A_{\textcolor{red}{#1}}%
        }%
    }

\begin{document}

With a subscript: $\MyMacro_{\pi}$ 

Without any subscript: $\MyMacro$ 

\end{document}