Reproduction

1. Make or gain shears in-world (survival gamemode)
2. Wear the shears down below the DurabilityInfo tool alert threshold
3. Observe as HUD does not show icon for shears

Reported by a Zyin's Hud user on a public server, confirmed by myself in single-player

This is an intended effect.

Even though shears are an item with durability that you can use, so are other things like fishing poles. But I decided not to incorporate these because these items really don't matter.

For shears, you won't lose any special enchants on them if they break. I remember using InvTweaks and it automatically unequipped my breaking shears and I thought it was an annoyance because then I'd have to go into my inventory and manually repair each shear to free up the space - but why? It's just 2 iron, not worth the effort imo to save them.

For fishing poles the durability goes down so incredibly slow that you can see when its breaking anyway.

On the contrary, both items are enchantable; Shears are subjectively more important in this regard.

Shears can be enchanted with Unbreaking 3, Efficiency 5 and Silk Touch but they are only enchantable by books. This means the player has to go through the extra effort to obtain these books and then anvil apply them to the shears,

As for fishing rods, as they are now enchantable with treasure-boosting enchants and said treasure include rare items such as saddles, there has been a significant rise in the usage of fishing rods (on the server I frequent, anyway). Although these are easier to enchant than shears.

If desired, I can work on a patch for both - I have been meaning to get around to testing the mouse side buttons idea for ItemSelector also

Making it work with shears/fishing poles is not something I'm willing to put effort into, but if you want to make a patch for it then go ahead. You may need to take the auto unequip functionality durability threshold into consideration as well since I think i used a static value of 20 which would be inappropriate for things like fishing poles.