Thanks @frossie. I managed to install v12.0 on Linux but not on OS X. This Linux version is now online. 
Here is my feedback. I could not follow the installation instructions exactly as stated in https://pipelines.lsst.io/install/newinstall.html#installation-set-up because the command:
curl -OL http://ls.st/8g2
creates a file named 8g2 and not newinstall.sh as I think is intended. I did instead:
curl -o newinstall.sh -L http://ls.st/8g2
In addition, executing newinstall.sh shows the warning below:
$ source newinstall.sh
LSST Software Stack Builder
=======================================================================
!!! This script differs from the official version on the distribution server.
    If this is not intentional, get the current version from here:
    http://sw.lsstcorp.org/eupspkg/newinstall.sh
<...>
The file in http://sw.lsstcorp.org/eupspkg/newinstall.sh is actually an older version of the one in http://ls.st/8g2.
It is nice to see that now you provide a demo tarball specific for each version.
Finally, I encountered an issue when building v12.0 on OS X Yosemite. The problem seems to be related to the package afw.  Here is the trace:
$ eups distrib install -t v12_0 lsst_apps
   <...>
   [ 39/62 ]  pex_config 12.0                                            done. 
   [ 40/62 ]  afw 12.0 ... 
***** error: from /cvmfs/lsst.in2p3.fr/software/darwin-x86_64/lsst-v12.0/EupsBuildDir/DarwinX86/afw-12.0/build.log:
The errors for vector batch interpolation with variance
worst mu error  1.30094019438e-05
worst sig2 error  2.09062343838e-07
The errors for vector batch interpolation without variance
worst mu error  1.30094019438e-05
The errors for vector add interpolation
worst mu error  0.000142775502779
worst sig2 error  3.84439639196e-07
The following tests failed:
/cvmfs/lsst.in2p3.fr/software/darwin-x86_64/lsst-v12.0/EupsBuildDir/DarwinX86/afw-12.0/afw-12.0/tests/.tests/testGaussianProcess.py.failed
1 tests failed
scons: *** [checkTestStatus] Error 1
scons: building terminated because of errors.
+ exit -4
eups distrib: Failed to build afw-12.0.eupspkg: Command:
	source /cvmfs/lsst.in2p3.fr/software/darwin-x86_64/lsst-v12.0/eups/bin/setups.sh; export EUPS_PATH=/cvmfs/lsst.in2p3.fr/software/darwin-x86_64/lsst-v12.0; (/cvmfs/lsst.in2p3.fr/software/darwin-x86_64/lsst-v12.0/EupsBuildDir/DarwinX86/afw-12.0/build.sh) >> /cvmfs/lsst.in2p3.fr/software/darwin-x86_64/lsst-v12.0/EupsBuildDir/DarwinX86/afw-12.0/build.log 2>&1 4>/cvmfs/lsst.in2p3.fr/software/darwin-x86_64/lsst-v12.0/EupsBuildDir/DarwinX86/afw-12.0/build.msg 
exited with code 252
I tried also to build using  export MACOSX_DEPLOYMENT_TARGET="10.9" and it fails also building afw, but the error message is different. I can provide it if that may help.