Shader cache has no means of automatically purging old entries. This means that the cache grows indefinitely unless manually deleted. The problem with manual cache purge is that there is no way to preserve the currently valid data, so full recompilation is required after it. When iterating changes on complex shaders and especially when touching include files that are used in multiple shaders, the cache size can quickly grow into gigabytes range. This can eventually lead into out of disk space situation.

https://forum.unity.com/threads/unity-editor-cache-folder-leak-ran-out-of-space-on-disk.1115794/