LaTeX: To refuse (u)pTeX and show an essential-esque error

gistfile1.txt

%%
%% This is file 'dcnoptex.sty'.
%%
%% Copyright (c) 2019 Takayuki YATO (aka. "ZR")
%%   GitHub:   https://github.com/zr-tex8r
%%   Twitter:  @zr_tex8r
%%
%% This package is distributed under the MIT License.
%%

%% package declarations
\NeedsTeXFormat{LaTeX2e}
\RequirePackage{expl3,xparse}
\ProvidesExplPackage{dcnoptex}{2019/10/01}{0.2}{quack}
\str_const:Nn \c_dcnoptex_pkgname { dcnoptex }

%--------------------------------------- main
\bool_if:nTF
  {
    \sys_if_engine_luatex_p:
    || \sys_if_engine_pdftex_p:
    || \sys_if_engine_xetex_p:
  }
  {
    \NewDocumentCommand \dcnoptexCheck { m } { }
  }
  {%>>>
    \NewDocumentCommand \dcnoptexCheck { m }
      {
        \msg_new:nnnn { #1 } { incompatible }
          {
            The~#1~package~is~currently~only~compatible~with~
            pdfTeX,~LuaTeX,~and~XeTeX!
          }
          {
            \ ~_____________________________\\
            \ (Quack!~Sorry~for~that.~Quack!)\\
            \ ~"""""""""""""""""""""""""""""\\
            \ ~\ ~\ ~\ ~\ ~\ ~\ ~\ ~\ ~\ ~\ \ ~\ ~\ ~\ ~\string\ \\
            \ ~\ ~\ ~\ ~\ ~\ ~\ ~\ ~\ ~\ ~\ \ ~\ ~\ ~\ ~\ ~>()_\\
            \ \ ~\ ~\ ~\ ~\ ~\ ~\ ~\ ~\ ~\ \ ~\ ~\ ~\ ~\ ~\ ~(__)__
          }
        \msg_error:nn { #1 } { incompatible }
      }
    \endinput
  }%<<<

%--------------------------------------- done
\endinput
%% EOF

For LaTeX package developers who can not read Japanese, it is nearly impossible to provide support for the (u)pTeX engines to the packages they are creating, maily because the (u)pTeX community will not provide any information on the engines in English. Thus it is rather sensible to refuse the (u)pTeX engines in advance and throw an error message when they are used.

Just insert the following code in your great LaTeX package:

\RequirePackage{dcnoptex}
\dcnoptexCheck{<package-name>}

That's all.