From the documentation: This does not appear to be the proper location for a custom icon library in 8.3. Did a search for material.svg and it is not found. I'm assuming this is still allowed. What folder should a custom library go into now?
Use a Custom Icon Repository
The materials icon repository is a great source for icons, but if you have a custom library of icons, you can set it up to be accessible by Ignition. All icon repositories are stored as SVG files within the Gateway's Installation Directory. A typical path to an icon file will look like:
Navigate to your install directory, and in the /data/config/resources/core/ folder, create a new folder com.inductiveautomation.perspective/ (if it doesn't already exist).
Within that folder create an icons/ folder.
Within that folder create a <repository name>/ folder.
Within that folder, create:
The spritesheet of your SVG, at whatever arbitrary filename you want, e.g. icons.svg.
A config.json file that contains a single entry, svgFileName, that contains the same name (icons.svg or whatever else you chose)
{
"svgFileName": "icons.svg"
}
A resource.json file that references both config.json and your custom icons SVG:
As an aside, I'm not sure if there's a rationale I'm not aware of for the indirection of the config.json resource. I'm going to make a case to drop it for final release, so you would just have a resource.json file pointing to the svg file (by whatever arbitrary name). There may be a complicated upgrade related reason we had to do things this way, though.