The library is no longer included beginning with VisualStudio 2012. You can download here though:
http://www.microsoft.com/en-us/download/details.aspx?id=35825
They updated it for 2022 in early February 2022. Content is similar across releases, so you only need to download the latest release.
The 2022 release has 12.1 MB of content and ~4000 icon types (SVG/PNG/XAML). This release got rid the folder structure entirely and got rid of ~5000 variants (like save_grey and RunTest_inverse). It also renamed many icons again (such as SaveStatusBar became StatusBarSave) and dropped the _16x suffix on all icons. The icons do look cleaner across the board and have a more consistent feel than past releases.
The 2019 release has 10.3 MB of content and 4085 icon types (SVG/PNG/XAML). This release got rid of many BMP icons, as well as the size variations used for overlays when making custom icons (SM/MD). It also added some handy icons (CloseAll, Trash, AlignCenter, AlignMiddle, SkipFile, SmartFilter, Sort, StatusReady, Target, UserGroup). It changed folder name conventions (such as AddRowToAbove became AddToRowAbove, column icons such as 1x and 2Columns became verbose such as TableColumn and SecondOfFourColumns, Silverlight/Azure prefixes were removed). The Save icon folder has 33 versions which together are 19.8KB, due to removed BMP and SM/MD types.
The 2017 release has 12.5 MB of content and 4013 icon types (BMP/SVG/PNG/XAML). This release largely abandoned the Adobe Illustrator (.ai) file type. Icons are still sorted into folders for the individual icon types that contain the different formats. Some icons have various size/resolution versions in each format up to 256x256 size and 32bit pixel depth. Most icons have different flavors such as grey/inverse/thin/thingrey/small/medium. For example, the Save icon folder has 40 versions which together are 24.9KB. The absence of the archive folder, the 2010 legacy icons, and the Adobe Illustrator (.ai) icons together makes this release nice and compact again.
The 2015 release has 2.9 GB of content and 4222 icon types (AI/BMP/SVG/PNG/XAML). This release started offering multiple flavors of each icon type such as grey/inverse/thin/thingrey/small/medium. Icons are now sorted into folders for the individual icon types that contain the different formats and flavors. Some icons have various size/resolution versions in each format up to 256x256 size and 32bit pixel depth. The Adobe Illustrator (.ai) file type adds most of the bulk to this release. For example the Save icon folder has 84 versions which together are 5.84MB, and the .ai files are 5.69MB of this.
The 2013 release has 285.6 MB of content and 2793 icon types (BMP/ICO/PNG/XAML). Icons were sorted by function group, then by image type (726 action, 1494 concept, 753 objects as counter by png). It also has 788 legacy icons for VS2010 and an archive folder with 2010 and 2012 icons. This release started offering multiple flavors of each icon type.
The 2012 release has 19.4 MB of content and 2658 icon types (BMP/ICO/PNG/XAML). Icons were sorted by function group, then by image type (1093 common, 722 action, 91 annotation, 752 objects as counted by PNG type). Most icons available as 16x16 size and 32bit pixel depth.