Deploy your plugin to the WordPress.org SVN plugin repository from your GitHub Repository.

deploy.sh

#! /bin/bash
#
# Script to deploy from Github to WordPress.org Plugin Repository
# A modification of a number of different sources:
# @link https://github.com/deanc/wordpress-plugin-git-svn
# @link https://github.com/GaryJones/wordpress-plugin-svn-deploy
# @link https://github.com/thenbrent/multisite-user-management/blob/master/deploy.sh
#
# Accompanying Tutorial Here:
# @link https://ericbusch.net/?p=106

# EDIT THIS LINE
SVNUSER="USERNAME_HERE" # Your WordPress.org SVN Username

# No editing required below this line.

#prompt for plugin slug
echo -e "Plugin Slug: \c"
read PLUGINSLUG

# main config, set off of plugin slug
CURRENTDIR=`pwd`
CURRENTDIR="$CURRENTDIR/$PLUGINSLUG"
MAINFILE="$PLUGINSLUG.php" # this should be the name of your main php file in the wordpress plugin

# git config
GITPATH="$CURRENTDIR/" # this file should be in the base of your git repository

# svn config
SVNPATH="/tmp/$PLUGINSLUG" # path to a temp SVN repo. No trailing slash required and don't add trunk.
SVNURL="http://plugins.svn.wordpress.org/$PLUGINSLUG/" # Remote SVN repo on WordPress.org, with no trailing slash

# Let's begin...
echo ".........................................."
echo
echo "Preparing to deploy WordPress plugin"
echo
echo ".........................................."
echo

# Check version in readme.txt is the same as plugin file
NEWVERSION1=`grep "^Stable tag" $GITPATH/readme.txt | awk -F' ' '{print $3}'`
echo "readme version: $NEWVERSION1"
NEWVERSION2=`grep "^Version" $GITPATH/$MAINFILE | awk -F' ' '{print $2}'`
echo "$MAINFILE version: $NEWVERSION2"

if [ "$NEWVERSION1" != "$NEWVERSION2" ]; then echo "Versions don't match. Exiting...."; exit 1; fi

echo "Versions match in readme.txt and PHP file. Let's proceed..."

cd $GITPATH
echo -e "Enter a commit message for this new version: \c"
read COMMITMSG
git commit -am "$COMMITMSG"

echo "Tagging new version in git"
git tag -a "$NEWVERSION1" -m "Tagging version $NEWVERSION1"

echo "Pushing latest commit to origin, with tags"
git push origin master
git push origin master --tags

echo
echo "Creating local copy of SVN repo ..."
svn co $SVNURL $SVNPATH

echo "Ignoring github specific files and deployment script"
svn propset svn:ignore "deploy.sh
README.md
.git
.gitignore" "$SVNPATH/trunk/"

#export git -> SVN
echo "Exporting the HEAD of master from git to the trunk of SVN"
git checkout-index -a -f --prefix=$SVNPATH/trunk/

#if submodule exist, recursively check out their indexes
if [ -f ".gitmodules" ]
then
echo "Exporting the HEAD of each submodule from git to the trunk of SVN"
git submodule init
git submodule update
git submodule foreach --recursive 'git checkout-index -a -f --prefix=$SVNPATH/trunk/$path/'
fi

echo "Changing directory to SVN and committing to trunk"
cd $SVNPATH/trunk/
# Add all new files that are not set to be ignored
svn status | grep -v "^.[ \t]*\..*" | grep "^?" | awk '{print $2}' | xargs svn add
svn commit --username=$SVNUSER -m "$COMMITMSG"

echo "Creating new SVN tag & committing it"
cd $SVNPATH
svn copy trunk/ tags/$NEWVERSION1/
cd $SVNPATH/tags/$NEWVERSION1
svn commit --username=$SVNUSER -m "Tagging version $NEWVERSION1"

echo "Removing temporary directory $SVNPATH"
rm -fr $SVNPATH/

echo "*** FIN ***"

Life saver this script!
I also ran into a few issues on my Mac due to spaces in the parent folders, as well as trying to find the version number in the PHP file. Here's some comments/fixes for future users....

1. Line 13 - Username case is important. Use whatever is shown as your username in your WordPress dot org profile.
2. Line 42 - wrap the $GITPATH variable in double quotes. The new line becomes: NEWVERSION1=`grep "^Stable tag" "$GITPATH"/readme.t...
3. Line 44 - Same issue as the one above, but I also wrap my plugin header the way WordPress shows it, ie with an * on a new line within the main opening and closing comment tags. As a result the script wasn't able to pick up the version number in the PHP file. To be clear, mine is written as * Version: 2.0. The '{print $2}' at the end of the line also needs to be adjusted for this. All in, the new line 44 looks like this: NEWVERSION2=`grep "^\* Version" "$GITPATH"/$MAINFILE | awk -F' ' '{print $3}'`
4. Line 51 - $GITPATH should also be wrapped in quotes - so the whole line should be: cd "$GITPATH"
5. Lastly, my Mac version no longer includes svn by default, so this will need to be installed. This SO answer saved the day, noting I went with the brew version on Catalina.

@nickpagz Cool, glad it helped and thanks for sharing your modifications!