TikZ version 3.1 is available on CTAN since January 5, 2019.

Christian Feuersaenger published the latest version on sourceforge on the same day.

The manual of version 3.0.1a which had 1161 pages now has 1282 pages.

What's new in this version?

Well according to the Announcement text it's

Re­lease 3.1 in­tro­duces the new an­i­ma­tions li­brary for HTML/SVG out­put and comes with tons of bug­fixes.

There's also the full changelog. (I borrowed this link from Henri Menke's comment)

• Hopefully, this will generate a lot of SVG and animation-related questions :-)) ! – AlexG Jan 7 at 19:23
• @AlexG Hopefully these SVG and animated-related questions will generate lot of your interesting answers -- I'm looking forward to reading them :) – user36296 Jan 7 at 19:57
• I must immediatly update! – vi pa Jan 8 at 12:33

I make this answer a Wiki in order to allow those who are aware of the new features of version 3.1 to make them public.

• A new pic type right angle is added. (page 561)

Pic type right angle=< A>--< B>--< C> This pic adds a drawing of a right angle to the current path. It works in the same way as angle pic.

\tikz
\draw (1,0,0) coordinate (A) -- (0,0,0) coordinate (B)
-- (0,0,1) coordinate (C)
(B) -- (0,1,0) coordinate (D)
pic [fill=gray,angle radius=4mm] {right angle = A--B--C}
pic [draw,red,thick,angle eccentricity=.5,pic text=$\cdot$]
{right angle = A--B--D};

• unfortunately "The Data Visualization Backend" which was not documented in version 3.0.1a is still not documented. (page 944)

• The 3d library that was available is now documented (page 557).

• In grid if the xstep or ystep is 0 or negative the corresponding lines are not drawn (p.157 of the documentation).

\begin{tikzpicture}
\draw (0,0) grid [xstep=.5,ystep=.75] (3,2);
\draw[thick,red] (0,0) grid [ystep=0] (3,2);
\end{tikzpicture}

• SVG Animations; compile with latex and dvisvgm --font-format=woff --exact --zoom=-1

\documentclass[dvisvgm]{standalone}
\usepackage{tikz}
\usetikzlibrary{animations}

\begin{document}

\tikz
\node :fill opacity = { 0s="1", 2s="0", begin on=click }
:rotate = { 0s="0", 2s="90", begin on=click }
[fill = blue!20, draw = blue, ultra thick, circle]
{Click me!};

\end{document}


Static SVG and animated SVG without user interaction (e. g. activation on click) are embedded into HTML with the <img> tag, e. g.:

<img src="https://url/of/some.svg" width="200"/>


The <img> tag also works on this site (TeX.SX). Unfortunately, Imgur does not allow SVG file upload, but file URLs to third-party sites can be used.

Animated SVG with user interaction and scripted SVG (as those produced with pkg animate) must be embedded into HTML using the <object> tag:

<object type="image/svg+xml" data="https://url/of/some.svg" width="200">

Unfortunately, Blog sites and StackExchange do not accept the <object> tag, mainly for security reasons. As a workaround, an <img> tag can be used as text of a link that redirects the browser to the file URL of the SVG on click. This was done for the example above. In Markdown syntax:
[<img src="https://url/of/some.svg" width="200"/>](https://url/of/some.svg)