Minecraft had the same launcher from it's creation to 2013, when Mojang replaced it with the new Minecraft launcher we see today. However, the old launcher continued to be used for those who didn't want to update to the new one. That is, until today. Because today we say goodbye to the original Minecraft launcher, thanks to a bug called Heartbleed.
The bug is called Heartbleed because it piggybacks on a feature called heartbeat, which affects specific versions of the widely-used OpenSSL library. OpenSSL, also known as Open Secure Sockets Layer, is what just about every site and online service uses to transfer sensitive information between you and the internet. Web logins, online banking, and even Minecraft's servers use OpenSSL. Heartbleed was discovered earlier today, and allows hackers to steal user names and passwords from just about any site on the internet (and even more than logins).
Mojang is one of thousands of companies affected by the bug, and earlier today all login servers were shut off to fix the bug. Around an hour later, the servers came back online with the bug fixed and a new Minecraft launcher update. Unfortunately, the launcher now uses a fixed security certificate that cannot be ported back to the older launcher. To avoid players' accounts being hacked, Mojang has decided to shut down the old Minecraft launcher.
This means that the old Minecraft launcher, as well as all custom launchers that use the old login protocol, are now disabled and cannot login to Minecraft. The new Minecraft launcher appears to be running normally now, however some custom launchers are having issues logging in.
Source: Twitter