Install postgis-2.3.2 with postgres@9.6

gistfile1.txt

brew install https://raw.githubusercontent.com/Homebrew/homebrew-core/aa049a47b218fda30f9a4a454119ae067a02cf50/Formula/postgis.rb

#pg_config --pkglibdir is useful for finding your $libdir
cp -a /usr/local/share/postgresql/extension/postgis* /usr/local/Cellar/postgresql@9.6/9.6.5/share/postgresql@9.6/extension/
cp -a /usr/local/lib/postgresql/postgis-2.3.so /usr/local/Cellar/postgresql@9.6/9.6.5/lib/postgis-2.3
cp -a /usr/local/lib/postgresql/rtpostgis-2.3.so /usr/local/Cellar/postgresql@9.6/9.6.5/lib/

Ok. I tried it, but had a number of issues. first of all I needed to use ln -s instead of cp-a.

Perfect thank you ! I had also an error not found /usr/local/opt/proj/lib/libproj.12.dylib, but I had /usr/local/opt/proj/lib/libproj.13.dylib. So i did ln -sf /usr/local/lib/libproj.13.dylib /usr/local/lib/libproj.12.dylib which is probably not a good thing to do, but it worked ^^

Updated commands for postgis 2.3 with postgres@9.6 9.6.9:

brew install https://raw.githubusercontent.com/Homebrew/homebrew-core/aa049a47b218fda30f9a4a454119ae067a02cf50/Formula/postgis.rb

#pg_config --pkglibdir is useful for finding your $libdir
ln -s /usr/local/share/postgresql/extension/postgis* /usr/local/Cellar/postgresql@9.6/9.6.9/share/postgresql@9.6/extension
ln -s /usr/local/lib/postgresql/postgis-2.3.so /usr/local/Cellar/postgresql@9.6/9.6.9/lib/postgis-2.3
ln -s /usr/local/lib/postgresql/rtpostgis-2.3.so /usr/local/Cellar/postgresql@9.6/9.6.9/lib/

# Fixes issues with missing 
brew reinstall gdal2

After execute brew install https://raw.githubusercontent.com/Homebrew/homebrew-core/aa049a47b218fda30f9a4a454119ae067a02cf50/Formula/postgis.rb

I'm receiving Postgresql 10.4 installed as dependency

Warning: postgis 2.4.4_1 is available and more recent than version 2.3.2.
==> Installing dependencies for postgis: postgresql
==> Installing postgis dependency: postgresql
==> Downloading https://homebrew.bintray.com/bottles/postgresql-10.4.high_sierra.bottle.tar.gz

I cannot create extension because:

hrm=# CREATE EXTENSION postgis;
ERROR:  could not load library "/usr/local/Cellar/postgresql@9.6/9.6.9/lib/postgis-2.3": dlopen(/usr/local/Cellar/postgresql@9.6/9.6.9/lib/postgis-2.3, 10): Library not loaded: /usr/local/opt/proj/lib/libproj.12.dylib
  Referenced from: /usr/local/Cellar/postgresql@9.6/9.6.9/lib/postgis-2.3
  Reason: image not found

gdal2 not found in brew

brew reinstall gdal2
Error: No available formula with the name "gdal2"

Tried to reinstall gdal, but it not solved problem of extension creation

Trying to install PostGIS to work with existing Postgres 9.6. The above installed PostgreSQL 11.1, and failed to install PostGIS. Got this:

==> Pouring postgis-2.3.2.high_sierra.bottle.tar.gz
Error: undefined method `undent' for #<String:0x00007f80feb5d228>
Please report this bug:
  https://docs.brew.sh/Troubleshooting
/Users/khommel/Library/Caches/Homebrew/Formula/postgis.rb:132:in `caveats'
/usr/local/Homebrew/Library/Homebrew/caveats.rb:17:in `caveats'
/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/2.3.0/forwardable.rb:202:in `empty?'
/usr/local/Homebrew/Library/Homebrew/formula_installer.rb:613:in `caveats'
/usr/local/Homebrew/Library/Homebrew/formula_installer.rb:664:in `finish'
/usr/local/Homebrew/Library/Homebrew/cmd/install.rb:330:in `install_formula'
/usr/local/Homebrew/Library/Homebrew/cmd/install.rb:259:in `block in install'
/usr/local/Homebrew/Library/Homebrew/cmd/install.rb:257:in `each'
/usr/local/Homebrew/Library/Homebrew/cmd/install.rb:257:in `install'
/usr/local/Homebrew/Library/Homebrew/brew.rb:89:in `<main>'