Joseph
I wanted a slanted node to have a fading shadow behind it.

Is there any way to:
* squeeze the shadow text a little bit;
* slant the shadow text (left or right) so as to simulate a different light origin?


\documentclass{article}
\usepackage{tikz}
\usepackage{tikzpagenodes}

\node[text = transparent!0, inner xsep = 0pt, outer xsep = 0pt] {#2};
\begin{tikzpicture}[baseline = (textnode.base)]
\node[inner sep = 0pt, outer sep = 0pt] (textnode) {\phantom{#2}};
(textnode.south west) rectangle (textnode.north east);%
\end{tikzpicture}%
}

\begin{document}

\tikzset{fading text/.style = {top color = white, bottom color=lightgray,
%middle color = green
}
}

\begin{tikzpicture}
\node[rotate=45,yshift=.7em] (text) {Neptune};
\end{tikzpicture}
\end{document}

user 3.14159
This is not a complete answer because I do not fully understand what you want to achieve, and because there might be alternatives that suit the problem better. The cover of the pgfmanual already contains one (but it also nests tikzpictures). This post is just to throw some slant and scale smaller than 1 into the game.

\documentclass{article}
\usepackage{tikz}

\node[text = transparent!0, inner xsep = 0pt, outer xsep =
0pt, #1] {#2};
\begin{tikzpicture}[baseline = (textnode.base)]
\node[inner sep = 0pt, outer sep = 0pt] (textnode) {\phantom{#2}};
(textnode.south west) rectangle (textnode.north east);%
\end{tikzpicture}%
}

\begin{document}

\tikzset{fading text/.style = {top color = white, bottom color=lightgray}}

\begin{tikzpicture}
\node[rotate=45,yshift=.8em] (text) {Neptune};
\begin{scope}[xshift=3cm]
\node[rotate=45,yshift=.8em] (text) {Neptune};
\end{scope}
\end{tikzpicture}
\end{document}

![Screen Shot 2020-09-07 at 11.33.39 AM.png](/image?hash=a96edf467daa311d18180cd268b03906ef53052dcb9fa97b4ee3de3ba535e477)

The right shadow is a bit shorter and a bit slanted. You can make this effect more pronounced by adjusting the xslant and yscale keys. Whether this is the most elegant way to get this result, I am not sure (actually I doubt it) but at least this code does no longer nest tikzpictures.

Alternatively one can mimick a shadow by projecting a diffuse light source on the xy plane in a 3d setup.

\documentclass[tikz,border=3mm]{standalone}
\usepackage{tikz-3dplot}
\node[text = transparent!0, inner xsep = 0pt, outer xsep =
0pt, #1] {#2};
\begin{tikzpicture}[baseline = (textnode.base)]
\node[inner sep = 0pt, outer sep = 0pt] (textnode) {\phantom{#2}};
(textnode.south west) rectangle (textnode.north east);%
\end{tikzpicture}%
}
\begin{document}
\tdplotsetmaincoords{70}{-70}
\foreach \Angle in {25,35,...,155,145,135,...,35}
{\begin{tikzpicture}[tdplot_main_coords,
nodes={inner sep=0pt,transform shape,text depth=0.3em},
declare function={Lx=3;Ly=3*cos(\Angle);Lz=3*sin(\Angle);}]
\path[tdplot_screen_coords,use as bounding box]
(-1,-1) rectangle (2,2);
\path (1,0,0) coordinate (ex)
(-Lx/Lz,-Ly/Lz,0) coordinate (ey);
\path[x={(ex)},y={(ey)},canvas is xy plane at z=0,above]