Soon after the release of 3.12.0 Heist league GGG updates tags on various items and modifiers. One type of those modifiers are now known as _herring tags. There are three in total _herring tags, each corresponding to a different color - red_herring, blue_herring and green_herring. Due to their tag names (and one of the Krillson's missions being Catch 2 Red Herrings), items with these mods are commonly refered to fishing related items.
So far, no specific use of these tags have been noticed. However, a significant number of fishing item drops occured in zones or by players containing at least one of these _herring tags. For example, out of 4 documented Replica Baited Breath drops in Heist, 2 of those occured in an of Miring blueprint, which has blue_herring tag.
While this is relationship is speculatory at best, it is a good idea to note all fishing-related items a player has during rare fishing event.