twemoji based on https://aur.archlinux.org/packages/ttf-twemoji/

75-twemoji.conf

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE fontconfig SYSTEM "fonts.dtd">
<fontconfig>

    <!--
    This config attempts to guarantee that colorful emojis from Twemoji will be displayed,
    no matter how badly the apps and websites are written.

    It uses a few different tricks, some of which introduce conflicts with other emoji fonts.

    If you wish to simultaneously use multiple emoji fonts, make sure to adapt this config.
    -->

    <!--
    This adds a generic family 'emoji',
    aimed for apps that don't specify specific font family for rendering emojis.
    -->
    <match target="pattern">
        <test qual="any" name="family"><string>emoji</string></test>
        <edit name="family" mode="assign" binding="same"><string>Twemoji</string></edit>
    </match>

    <!--
    This adds Twemoji as a final fallback font for the default font families.
    In this case, Twemoji will be selected if and only if no other font can provide a given symbol.

    Note, usually other fonts will have some glyphs available (especilly Symbola font),
    causing some emojis to be black&white and ugly.
    -->
    <match target="pattern">
        <test name="family"><string>sans</string></test>
        <edit name="family" mode="append"><string>Twemoji</string></edit>
    </match>

    <match target="pattern">
        <test name="family"><string>serif</string></test>
        <edit name="family" mode="append"><string>Twemoji</string></edit>
    </match>

    <match target="pattern">
        <test name="family"><string>sans-serif</string></test>
        <edit name="family" mode="append"><string>Twemoji</string></edit>
    </match>

    <match target="pattern">
        <test name="family"><string>monospace</string></test>
        <edit name="family" mode="append"><string>Twemoji</string></edit>
    </match>

    <!--
    It seems Symbola is often selected as a fallback font before Twemoji.
    This will try to remove Symbola from the list of fallback fonts.
    -->
    <selectfont>
        <rejectfont>
            <pattern>
                <patelt name="family">
                    <string>Symbola</string>
                </patelt>
            </pattern>
        </rejectfont>
    </selectfont>

    <!--
    Recognize alternative ways of writing Twemoji family name.
    -->
    <match target="pattern">
        <test qual="any" name="family"><string>Emoji One</string></test>
        <edit name="family" mode="assign" binding="same"><string>Twemoji</string></edit>
    </match>

    <match target="pattern">
        <test qual="any" name="family"><string>Twemoji Color</string></test>
        <edit name="family" mode="assign" binding="same"><string>Twemoji</string></edit>
    </match>

    <match target="pattern">
        <test qual="any" name="family"><string>Twemoji Mozilla</string></test>
        <edit name="family" mode="assign" binding="same"><string>Twemoji</string></edit>
    </match>

    <!--
    Use Twemoji when other popular fonts are being specifically requested.

    It is quite common that websites would only request Apple and Google emoji fonts, and then fallback to b&w Symbola.
    These aliases will make Twemoji be selected in such cases to provide good-looking emojis.
    -->
    <match target="pattern">
        <test qual="any" name="family"><string>Apple Color Emoji</string></test>
        <edit name="family" mode="assign" binding="same"><string>Twemoji</string></edit>
    </match>

    <match target="pattern">
        <test qual="any" name="family"><string>Segoe UI Emoji</string></test>
        <edit name="family" mode="assign" binding="same"><string>Twemoji</string></edit>
    </match>

    <match target="pattern">
        <test qual="any" name="family"><string>Segoe UI Symbol</string></test>
        <edit name="family" mode="assign" binding="same"><string>Twemoji</string></edit>
    </match>

    <match target="pattern">
        <test qual="any" name="family"><string>Noto Color Emoji</string></test>
        <edit name="family" mode="assign" binding="same"><string>Twemoji</string></edit>
    </match>

    <match target="pattern">
        <test qual="any" name="family"><string>NotoColorEmoji</string></test>
        <edit name="family" mode="assign" binding="same"><string>Twemoji</string></edit>
    </match>

    <match target="pattern">
        <test qual="any" name="family"><string>Android Emoji</string></test>
        <edit name="family" mode="assign" binding="same"><string>Twemoji</string></edit>
    </match>

    <match target="pattern">
        <test qual="any" name="family"><string>Noto Emoji</string></test>
        <edit name="family" mode="assign" binding="same"><string>Twemoji</string></edit>
    </match>

    <match target="pattern">
        <test qual="any" name="family"><string>Twitter Color Emoji</string></test>
        <edit name="family" mode="assign" binding="same"><string>Twemoji</string></edit>
    </match>

    <match target="pattern">
        <test qual="any" name="family"><string>EmojiOne</string></test>
        <edit name="family" mode="assign" binding="same"><string>Twemoji</string></edit>
    </match>

    <match target="pattern">
        <test qual="any" name="family"><string>Twemoji Mozilla</string></test>
        <edit name="family" mode="assign" binding="same"><string>Twemoji</string></edit>
    </match>

    <match target="pattern">
        <test qual="any" name="family"><string>TwemojiMozilla</string></test>
        <edit name="family" mode="assign" binding="same"><string>Twemoji</string></edit>
    </match>

    <match target="pattern">
        <test qual="any" name="family"><string>EmojiTwo</string></test>
        <edit name="family" mode="assign" binding="same"><string>Twemoji</string></edit>
    </match>

    <match target="pattern">
        <test qual="any" name="family"><string>Emoji Two</string></test>
        <edit name="family" mode="assign" binding="same"><string>Twemoji</string></edit>
    </match>

    <match target="pattern">
        <test qual="any" name="family"><string>EmojiSymbols</string></test>
        <edit name="family" mode="assign" binding="same"><string>Twemoji</string></edit>
    </match>

    <match target="pattern">
        <test qual="any" name="family"><string>Symbola</string></test>
        <edit name="family" mode="assign" binding="same"><string>Twemoji</string></edit>
    </match>
</fontconfig>

default.nix

{ stdenv, fetchurl, rpmextract }:

let
  fedRel = "1.fc30";
in
stdenv.mkDerivation rec {
  name = "twemoji-${version}";
  version = "11.2.0";

  src = fetchurl {
    url = "https://archives.fedoraproject.org/pub/fedora/linux/development/rawhide/Everything/x86_64/os/Packages/t/twitter-twemoji-fonts-${version}-${fedRel}.noarch.rpm";
    sha256 = "c6487c14f2e719019f539613267b70bca920afee1b06ce05bd062ee2af98dc2c";
  };

  buildInputs = [ rpmextract ];

  unpackPhase = ''
    rpmextract $src
  '';

  installPhase = ''
    install -Dm644 usr/share/fonts/twemoji/Twemoji.ttf \
      $out/share/fonts/truetype/twemoji.ttf
    install -Dm644 ${./75-twemoji.conf} $out/etc/fonts/conf.d/75-twemoji.conf
    install -Dm644 ${./75-twemoji.conf} $out/etc/fonts/2.11/conf.d/75-twemoji.conf
  '';
}