Based on 2024 PHB rules, what is the applicability of twinned spell metamagic to chromatic orb?  I have read some discourse on the topic and have my own perspective (no) but am interested in others' thoughts and their rationale.    

In the 2024 rules you cannot twin it anymore. 

There are like 20 spells you can twin. And no I do not think chromatic orb is not one of them. It has to specify an additional target when you up cast it.  The leap is not an additional target, imo.

The new twin with its cut rate costs is powerful if you are learning those spells, its just kind of boring. I wish it also worked with spells with additional attacks when you upcast them, like magic missile. It wouldn't add that many spells but it would expand its usefulness past crowd control and a couple utility spells.

I could see this go either way as updating the spell does in fact open the possibility of one more target but you would still have to get a pair on the dice and make your attack roll.

Per the 2024 rules, Chromatic Orb is not eligible for Twinned Spell. 

Also, I believe the 2024 rules for Chromatic Orb would disqualify it from Twinned Spell defined by the 2014 rules. 

Twinned Spell has two criteria for eligibility:

1) Can it be up cast? 

Chromatic Orb - Yes.

2) Does up casting the Spell enable the spell to target an additional target? 

Chromium Orb - Yes. 

Can you Twin Spell Chromatic Orb? Of course you can. 

Does Twin Spell Chromatic Orb allow me to cast 2 Chromatic Orbs? No. All Twinned Spell allows you to do is up cast the spell using a sorcery point instead of a higher level spell slot. So Twin Spell a 1st level Chromatic Orb turns it into a 2nd level Chromatic Orb and so on. 

Is this Balanced? Yes. An up cast Hold Person is way stronger than an upcast Chromatic Orb. 

According to the rules Chromatic Orb cannot be upcast. I'm surprised how many voted yes. Let's go over the wording of the metamagic and the spell:

Twinned Spell

When you cast a spell, such as Charm Person, that can be cast with a higher-level spell slot to target an additional creature, you can spend 1 Sorcery Point to increase the spell's effective level by 1.

Chromatic Orb

Using a Higher-Level Spell Slot. The damage increases by 1d8 for each spell slot level above 1. The orb can leap a maximum number of times equal to the level of the slot expended, and a creature can be targeted only once by each casting of this spell.

It doesn't state anywhere in Chromatic Orb that it can target an additional creature using a higher-level slot. It increases the amount of times it can leap.

Spells affected by Twinned Spell have a very specific wording, like Charm Person

Charm Person

Using a Higher-Level Spell Slot. You can target one additional creature for each spell slot level above 1.

This is the trigger for Twinned Spell, that Chromatic Orb doesn't have. The wording of Twinned Spell and the spells it can affect is very clear.  Thematically, Twinned Spell metamagic makes a second casting of a spell that targets one creature, only this time it hits two creatures.

Other examples:

Etherealness cannot be Twinned, despite providing additional targets when upcast. Since the wording of Twinned Spell asks for a very specific trigger

Using a Higher-Level Spell Slot. You can target up to three willing creatures (including yourself) for each spell slot level above 7. The creatures must be within 10 feet of you when you cast the spell.

Chain Lightning cannot be twinned either, despite causing extra bolts to leap, again, it's not the trigger that Twinned Spell uses

Using a Higher-Level Spell Slot. One additional bolt leaps from the first target to another target for each spell slot level above 6.

Scorching Ray cannot be twinned despite creating extra rays when upcast.

Using a Higher-Level Spell Slot. You create one additional ray for each spell slot level above 2.

Sorcerer Spells with the specific wording required are:

Hold Monster

Charm Monster

Banishment

Fly

Spider Climb

Invisibility

Hold Person

Enhance Ability

Blindness/Deafness

Charm Person