I've been checking out the docs for the base layer, and I think there are some possibilities for improvement regarding the compatibility options:

1. The version option for setting the compatibility layer is only available for LaTeX. Presumably it should be available for plain and ConTeXt, too. Although I currently only use LaTeX, so I don't care about this much.
2. Not all possible version options are documented in the manual, latest isn't, and neither is 1.18
3. Looking at the source https://github.com/pgf-tikz/pgf/blob/master/tex/latex/pgf/basiclayer/pgf.sty#L18-L24, in particular line 22, it seems that there may be a typo, if the users specified 0.96, presumably they should be given 0.96 and not 1.18?
4. I believe one more option should be available, something like 3.1. It would currently of course be equivalent to latest, but would keep backwards compatibility with version 3.1 in the future.

In reply to @hmenke:matrix.org
These compatibility layers have been completely abandoned for PGF.

In reply to @hmenke:matrix.org
Also why do you think there should be a 3.1 compatibility option? Have any basic layer commands been removed in 3.1?

In reply to @hmenke:matrix.org
Also why do you think there should be a 3.1 compatibility option? Have any basic layer commands been removed in 3.1?

In reply to @nsajko:matrix.org
In the manual, the definition of \pgfcoordinate, and a few other places in the manual, reference the shape coordinate. However, shape coordinate doesn't seem to be documented.

OK I found the answer to this, the shape doesn't have it's own entry like other shapes, but there's this paragraph which explains that "coordinate" is confusingly a 2D vector/point:

The three shapes rectangle, circle, and coordinate are always defined and no library needs to be loaded for them. While the coordinate shape defines only the center anchor, the other two shapes define a standard set of anchors.

In reply to @nsajko:matrix.org
In principle I'm not opposed to reading the source to find out stuff like this, but in this case I don't know where to look.

Just in case you're still curious about this.
https://github.com/pgf-tikz/pgf/blob/3.1.10/tex/generic/pgf/modules/pgfmoduleshapes.code.tex#L412-L442

The doc for basic layer is incomplete. I think some basic layer commands are even not mentioned in the pgfmanual.

In reply to @nsajko:matrix.org
Unrelated question: suppose I have a node created by a third-party LaTeX package: how do I find out the shape of the node without reading the source of the third-party latex package?

For node <x>, it's shape is stored in control sequence \pgf@sh@ns@<x>. For example, the example below typesets macro:->rectangle, the shape of node current page

\documentclass{article}
\usepackage{tikz}
\usepackage{etoolbox}

\begin{document}
\ttfamily
\csmeaning{pgf@sh@ns@current page}
\end{document}

I find this with the help of how \pgfnodealias is defined. Suppose \pgfnodealias covers all the info a node name corresponds to.

In reply to @nsajko:matrix.org
Unrelated question: suppose I have a node created by a third-party LaTeX package: how do I find out the shape of the node without reading the source of the third-party latex package?

For node <x>, it's shape is stored in control sequence \pgf@sh@ns@<x>. For example, the example below typesets macro:->rectangle, the shape of node current page

\documentclass{article}
\usepackage{tikz}
\usepackage{etoolbox}

\begin{document}
\ttfamily
\csmeaning{pgf@sh@ns@current page}
\end{document}

I find this with the help of how \pgfnodealias is defined. Suppose \pgfnodealias covers all the info a node name corresponds to.