Officially you should not remove this folder, as per the MSDN forum mentioned in Koenyn's answer.
However it seems to be "safe" (for some definition of safe) to free up some space by moving certain parts of it to another disk e.g. an external HDD.
It appears that these installers are placed for potential future use on-demand if you want to use certain features of Visual Studio - if you know you're never ever going to want to use certain features of Visual Studio in future, then you could move certain items elsewhere to free up space on your C: drive.
For example after installing Visual Studio 2015 I moved the following to a separate drive:
- AndroidSDK*
- Win10_Emulator_*
- VSEmu_Android*
because I knew I wouldn't be doing any mobile dev for Android or Windows Phone.
This freed up 8GB out of the 16GB present in my VisualStudioSecondaryInstaller folder.
Yes, but I currently don't want to test deinstallation, which may be the only thing that will stop working. – Destroy666 – 2015-10-10T19:16:26.523
Destroy666, if something doesn't work, you can go to the install and do "repair", which will put things back. – zumalifeguard – 2015-10-21T05:06:57.197
I know, but I might not have the VS2015 installer file at that point. Anyways, I've already removed the biggest installers (ADK, NDK, Xamarin) and noticed no issue so far. – Destroy666 – 2015-10-21T18:17:00.657