Never seen one deleted for lack of volume - they have a date specific for expiration so I think that prevents deletion for inactivity, could be wrong but I haven't lost any thru deletion except for mergers and expirations, Bankruptcy will also generally kill a warrant or give a pro-rata part of the newco warrants.
I don't know why some drop to pinks, some may be the result of uplisting to Nasdaq and the warrants get left behind. If you find an answer I'd love to hear it - Kinda the whole purpose of this board is to share info to make better decisions on warrant purchasing.
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