Merge pull request #1582 from rwhogg/sha1-gone

docs/Checksum_Deprecation.md: Note that SHA-1 now blocks installation

docs/Checksum_Deprecation.md

@@ -5,23 +5,18 @@ integrity verification. Since then every formulae under the Homebrew organisatio
 has been moved onto _SHA256_ verification; this includes both source packages
 and our precompiled packages (bottles).
 
-We also stopped supporting _MD5_ entirely. It was removed from core formulae in 2012 but until April 2015 if you tried to install a formula still using an
-_MD5_ checksum Homebrew wouldn't actively stop you.
+We have stopped supporting _SHA1_ and _MD5_ entirely.
+_MD5_ checksums were removed from core formulae in 2012 but until April 2015
+if you tried to install a formula still using one Homebrew wouldn't actively stop you.
 
-On _SHA1_ we added a `brew audit` check that flags _SHA1_ checksums as deprecated
-and requests that you use _SHA256_.
-
-We saw positive ecosystem engagement on moving from _MD5_ & _SHA1_ to the recommended _SHA256_ and thanks to that we're in a strong position to move forwards.
-
-## Moving forwards on SHA1.
+We removed _SHA1_ support in **November 2016**,
+21 months after we started warning people to move away from it for verification.
+This is enforced in the same way _MD5_ is, by blocking the installation of that
+individual formula until the checksum is migrated.
 
 From March 20th 2016 we've stepped up the visibility of that notification & you'll start
 seeing deprecation warnings when installing _SHA1_-validated formula.
 If you see these please consider reporting it to where the formula originated.
 
-We're targeting **the end of September 2016** for _SHA1_ support removal,
-19 months after we started warning people to move away from it for verification.
-This will be enforced in the same way _MD5_ is today, by blocking the installation of that individual formula until the checksum is migrated.
-
-This means prior to that date custom taps, local custom formulae, etc
-need to be migrated to use _SHA256_.
+This means custom taps, local custom formulae, etc need to be migrated to use
+_SHA256_ before you can install them.