From the author's README on Thunderstore

CASTE Graffiti Board

From Half Life. The "Caste" graffiti is found sprayed near Black Mesa East and other places such as Matt's home, depicting a soldier (not identified as belonging to either the Combine or Resistance) clutching a baby next to the word "Caste". This artwork was illustrated by Eric Kirchmer.

In Game

Board selection.

Credits

By Blauntd. Built with ReSkate Studio.

Install CASTE Board

You need ReSkate and your own copy of skate. on Steam.

Open ReSkateLauncher.exe and go to MODS.
Search for CASTE Board and click install. The launcher also installs anything it depends on.
Press PLAY. In game, the MODS tab of the ReSkate menu (Insert) turns it on and off.

Or install it by hand: download the .zip and drop it on the launcher window, or unzip it into its own folder in Mods\ beside Skate.exe.

Maps show up in the ReSkate menu's level list. To join a server running a custom map, you need the same map installed. If the game won't load after installing, see known issues.

skatemods doesn't host or review this mod. The download comes straight from Thunderstore. Only install mods you trust, because scripts can run code.