Why is something that'll probably need to be answered whenever the IOC gives their reasonings but this Should be the list of what things get lumped into that haven't been listed yet based on other checklist, listing by common names in IOC:
IOC ONLY (recognized today by IOC, but not HBW/BLI nor eBird/Clements, lumped in Avilist)
Wedge-tailed Sabrewing lumped with Curve-winged Sabrewing
Tricolored Grebe lumped with Little Grebe
Dimorphic Egret lumped with Little Egret
White-backed Stilt lumped with Black-necked Stilt
Cabot's Tern lumped with Sandwich Tern
White-breasted, Plain-breasted, and Rufous-thighed Hawk lumped with Sharp-shinned Hawk
Cape Verde Buzzard lumped with Long-legged Buzzard
Torotoroka Scops Owl lumped with Rainforest, maybe becoming Madagascar Scops Owl
Niau Kingfisher lumped with Mangareva, maybe becoming Tuamotu Kingfisher
Chilean Elaenia lumped with White-crested Elaenia
Coopman's Tyrannulet lumped with Golden-faced Tyrannulet
Loja Tyrannulet lumped with Peruvian Tyrannulet
Scarlet and Vermillion Flycatcher are getting lumped, might switch the common name for taxa
Huon Bowerbird with MacGregor's Bowerbird
Western Whistler with Australian Golden Whistler
Cinnamon-breasted Whistler with John's Whistler
Tagula Manucode with Glossy-mantled Manucode
Inca Jay with Green Jay
Indian Jungle and Eastern Jungle Crows with Large-billed Crow
Benguela Long-billed Lark with Karoo Long-billed Lark
Maghreb Lark with Crested Lark
Anjouan Brush Warbler with Malagasy Brush Warbler
Gray-backed and Green-backed Camaroptera
Sharpe's Greenbul with Yellow-streaked Greenbul
Kakamega Greenbul with Shelley's Greenbul
Somali, Dodson's, and Dark-capped Bulbul with Common Bulbul
Burmese Bushtit (and Black-browed Bushtit, also on HBW) lumped with Rufous-fronted Bushtit, not sure what'll come of eventual common name
Flores Leaf Warbler with Timor Leaf Warbler
Leaflitter and Glissando Babbler lumped with Mourning Babbler
Bornean Swamp Babbler with Malayan Swamp Babbler
Enggano Hill Myna lumped with Common Hill Myna
Mediterranean Flycatcher with Spotted Flycatcher
Madagascar Stonechat with African Stonechat
Ruaha Chat with Arnot's Chat
Basalt and Maghreb Wheatear with Mourning Wheatear
Striped Flowerpecker with Thick-billed Flowerpecker
Whyte's with Ludwig's Double-collared Sunbird
Royal with Red-headed Parrotfinch
Bonin with Oriental Greenfinch
Goldman's (and Audubon's Warbler also on HBW) with Myrtle Warbler
Tooth-billed (and Hepatic, also on HBW) with Red Tanager
Lemon-rumped Tanager with Flame-rumped Tanager
IOC+HBW/BLI ONLY (recognized today by IOC and HBW/BLI not eBird/Clements, lumped in Avilist)
Green-winged Teal lumped with Eurasian Teal
Fernando Po with African Black Swift
Grey-tailed with White-throated Mountaingem
Cinnamon-sided with Green-fronted Hummingbird
Kai Coucal with Pheasant Coucal
Ryukyu Green Pigeon with Taiwan Green Pigeon
Campbell with Black-browed Albatross
Yellow-billed with Black Kite
Lesser Sooty Owl with Greater Sooty Owl
Northern, Guatemalan, and Baja Pygmy Owl with Mountain Pygmy Owl
Annobon with African Scops Owl
Usambiro Barbet with D'Arnaud's Barbet
Wagler's and Blue-throated Toucanet with Emerald Toucanet
Black-throated Toucanet with White-throated Toucanet
Stripe-billed Aracari with... Probably Collared Aracari, not sure here
Citron-throated Toucan with Channel-billed Toucan
Little Spotted Woodpecker with Little Green Woodpecker
Bronze-winged with Golden-olive Woodpecker
Sumatran with Grey-headed Woodpecker
Cliff with Monk Parakeet (Depending on SACC?)
Palkachupa Cotinga with Swallow-tailed Cotinga
Yellow-cheeked with Green-backed Becard
Silvery-flanked Antwren with White-flanked Antwren
Biak Fantail with Northern Fantail
Brown-tailed with Yellow-breasted Apalis
Black-fronted with Green-fronted White-eye
Oya Tabu with Capped White-eye
Violet-hooded Starling with Metallic Starling
Australian with (Sunda) Zebra Finch
Chinese Long-tailed Rosefinch with Siberian Long-tailed Rosefinch
Sooty, Thick-billed, and Slate-colored Fox Sparrow with Red Fox Sparrow
Merida with Moustached Brushfinch
Chiriqui Yellowthroat with Olive-crowned Yellowthroat
Roraiman Warbler with Two-banded Warbler
Yellow-tufted with Black-faced Dacnis (depending on SACC?)
White-browed with Black-capped Hemispingus
IOC+CLE (recognized today by IOC and eBird/clements, not by HBW/BLI, lumped in Avilist)
Heard Island, Crozet, Macquarie, Antarctic, South Georgia, and Kerguelen Shag lumped with Imperial Shag
Moluccan and Manus Masked Owl lumped with Australasian Masked Owl
Black-capped, Huon, Northern, and Arfak Catbird lumped with Black-eared Catbird
Sepik-Ramu, Rufous, Tagula, Mamberano, and Waigeo Shrikethrushes with Arafura Shrikethrush
Willard's Sooty Boubou with Mountain Sooty Boubou
With the Philippine Jungle Crow, I suspect Large-billed Crow isn't staying lumped like that in the future, I think eBird 2024 Taxonomy update even said to expect splits there in the future but that was the most obvious one for now but yeah