I just leave it in the hive...
Reading the old books.... Dadant, Langstroth and others.. they claim just what Mr. Bush said.. they use it. The older it is the stronger.. was wondering myself if that was a wise thing to do. Now I know.
My bees have been foundationless since 2007, so for myself and my bees it would depend on how close we were to the nearest 'industrial' agriculture area. Fortunately we are surrounded by lakes and forest. Before man-made pesticides, fungicides and herbicides came around, black comb was considered gold by beekeepers. I and many others no longer feel that way after numerous studies related to contaminated comb, to include most of the foundation that is sold in the U.S. as it is loaded with nasty stuff (remember, its basically used old comb).
I try to remove 'old' brood comb every 5 years by coding my frames with the year they were placed, but after five years they are pretty dark, some after just three.
We harvest most of it, clean it up as best as we can and make candles for friends and family. Sorry, but I don't understand just throwing it away.