glass as a tag is meant to be used for when the glass is visible as an object in and of itself, such as with being shattered or being the focus of a reflected image. Having this tag show up in something like post #11050078 or post #11049539 would effectively create a padding tag and require us to create an additional tag just for posts where glass is visible as an object.
These tags tend to turn to shit because they are too generic, they are actually proxy tags for something more specific (such as looking at window/door glass that has a reflection, or stained glass, or broken glass, or glass walls/floors, or someone being against the glass), especially when we don't already have a more specific tag.
Honestly it might need to be deprecated in favor of things like glass floor/glass surface and other specific tags, I don't think we want glass to just be 99% window pictures either. Other material tags like wood and metal have or had the same issue, and in recent years we've tended to get rid of material/pattern tags that are too broad, like striped and checkered.