How to give horizontal connections in a tree using tikz & forest

Question

I am looking for an extension of the existing question and discussion

\documentclass[tikz,border=10pt]{standalone}
\usepackage[edges]{forest}
\usetikzlibrary{arrows.meta,shadows.blur}
\begin{document}
\colorlet{myyellow}{yellow!50}
\begin{forest}
colour me/.style={top color=#1!75, bottom color=#1, draw=#1, thick, blur shadow, rounded corners},
for tree={
edge=-Latex,
font=\sffamily,
},
where level=1{
for tree={
  folder,
  grow'=0,
},
edge path'={(!u.parent anchor) -- ++(0,-15pt) -| (.child anchor)},
}{},
before typesetting nodes={for tree={content/.wrap value={\strut #1},},
if={isodd(n_children("!r"))}{
  for nodewalk/.wrap pgfmath arg={{fake=r,n=#1}{calign with current edge}}{int((n_children("!r")+1)/2)},
}{},
tempcounta/.max={level}{tree},
for tree={
  colour me/.wrap pgfmath arg={cyan!#1!myyellow}{100*((tempcounta)-level())/(tempcounta)}
}
}
[Project Plan
[Objective-A
  [A1]
  [A2]
  [A3]
  [A4]
]
[Objective-B
  [B1]
  [B2]
  [B3]
  [B4[Extra]]
 [ \hspace{0.0cm} Data Analysis, no edge, colour me=red]
]
[Objective-C
  [C1]
  [C2]
  [C3]
  [C4]
  [C5]
]
[Objective-D
  [D1]
  [D2]
  [D3]
  [D4]
]
]
\end{forest}
\end{document}

Answer 1

You can't add these to the tree specification, as they make it a non-tree, but you can use regular TikZ to add them, optionally via the tikz key.

\documentclass[tikz,border=10pt]{standalone}
\usepackage[edges]{forest}
\usetikzlibrary{arrows.meta,shadows.blur}
\begin{document}
\colorlet{myyellow}{yellow!50}
\begin{forest}
  colour me/.style={top color=#1!75, bottom color=#1, draw=#1, thick, blur shadow, rounded corners},
  for tree={edge=-Latex, font=\sffamily,},
  where level=1{
    for tree={folder, grow'=0, },
    edge path'={(!u.parent anchor) -- ++(0,-15pt) -| (.child anchor)},
    if nodewalk valid={p}{tikz+={\draw [-Latex] (!p) -- (); } }{},
  }{},
  before typesetting nodes={for tree={content/.wrap value={\strut #1},},
    if={isodd(n_children("!r"))}{
      for nodewalk/.wrap pgfmath arg={{fake=r,n=#1}{calign with current edge}}{int((n_children("!r")+1)/2)},
    }{},
    tempcounta/.max={level}{tree},
    for tree={
      colour me/.wrap pgfmath arg={cyan!#1!myyellow}{100*((tempcounta)-level())/(tempcounta)}
    },
  },
  [Project Plan
  [Objective-A, tikz+={\draw [-Latex] (.west) -- ++(-5pt,0) |- (da.west);  }
    [A1]
    [A2]
    [A3]
    [A4]
  ]
  [Objective-B
    [B1]
    [B2]
    [B3]
    [B4[Extra]]
   [Data Analysis, no edge, colour me=red, name=da]
  ]
  [Objective-C
    [C1]
    [C2]
    [C3]
    [C4]
    [C5]
  ]
  [Objective-D, tikz+={\draw [-Latex] (.east) -- ++(5pt,0) |- (da.east);  }
    [D1]
    [D2]
    [D3]
    [D4]
  ]
]
\end{forest}
\end{document}

EDIT

If you want the root to be more centralised, you might centre it either between the first and last child or between the second and third. This code takes the former option, but could be easily adapted to implement the latter.

\documentclass[tikz,border=10pt]{standalone}
\usepackage[edges]{forest}
\usetikzlibrary{arrows.meta,shadows.blur}
\begin{document}
\colorlet{myyellow}{yellow!50}
\begin{forest}
  colour me/.style={top color=#1!75, bottom color=#1, draw=#1, thick, blur shadow, rounded corners},
  for tree={edge=-Latex, font=\sffamily,},
  where level=1{
    for tree={folder, grow'=0, },
    edge path'={(!u.parent anchor) -- ++(0,-15pt) -| (.child anchor)},
    if nodewalk valid={p}{tikz+={\draw [-Latex] (!p) -- (); } }{},
  }{},
  before typesetting nodes={
    for tree={content/.wrap value={\strut #1},},
    if={isodd(n_children("!r"))}{
      for nodewalk/.process={Ow+nw{!r.n children}{(#1+1)/2}{fake=r,n=#1}{calign with current edge}},
    }{},
    tempcounta/.max={level}{tree},
    for tree={
      colour me/.process={ROw2+Pw{tempcounta}{level}{100*(#1-#2)/#1}{cyan!#1!myyellow}},
    },
  },
  before drawing tree={%
    tempdima'=0pt,
    for nodewalk={
      fake=r, 1, tempdima/.option=x, tempdima+/.option=min x, fake=r, l, tempdima+/.option=x, tempdima+/.option=max x
    }{},
    tempdima*=.5,
    x/.register=tempdima,   
  }
  [Project Plan, 
  [Objective-A, tikz+={\draw [-Latex] (.west) -- ++(-5pt,0) |- (da.west);  }
    [A1]
    [A2]
    [A3]
    [A4]
  ]
  [Objective-B
    [B1]
    [B2]
    [B3]
    [B4[Extra]]
   [Data Analysis, no edge, colour me=red, name=da]
  ]
  [Objective-C
    [C1]
    [C2]
    [C3]
    [C4]
    [C5]
  ]
  [Objective-D, tikz+={\draw [-Latex] (.east) -- ++(5pt,0) |- (da.east);  }
    [D1]
    [D2]
    [D3]
    [D4]
  ]
]
\end{forest}
\end{document}