LGPE has all the Kanto Pokemon plus Meltan and Melmetal, while SwSh has a large collection of Pokemon spanning the first 8 generations. However, for all of the Gen 1-7 Pokemon unobtainable in both games, Showdown uses the menu icons from Gens 6 and 7, meaning that the only Pokemon with custom-made menu icons on Showdown are those introduced in PLA and Gen 9.
But here's a list of all Pokemon unobtainable in SwSh and LGPE, anyway:
Chikorita
Bayleef
Meganium
Cyndaquil
Quilava
Typhlosion
Totodile
Croconaw
Feraligatr
Sentret
Furret
Ledyba
Ledian
Spinarak
Ariados
Mareep
Flaaffy
Ampharos
Hoppip
Skiploom
Jumpluff
Aipom
Sunkern
Sunflora
Yanma
Murkrow
Misdreavus
Unown
Girafarig
Pineco
Forretress
Gligar
Snubbull
Granbull
Teddiursa
Ursaring
Slugma
Magcargo
Houndour
Houndoom
Phanpy
Donphan
Stantler
Smeargle
Poochyena
Mightyena
Wurmple
Silcoon
Beautifly
Cascoon
Dustox
Taillow
Swellow
Surskit
Masquerain
Shroomish
Breloom
Slakoth
Vigoroth
Slaking
Makuhita
Hariyama
Nosepass
Skitty
Delcatty
Meditite
Medicham
Plusle
Minun
Volbeat
Illumise
Gulpin
Swalot
Numel
Camerupt
Spoink
Grumpig
Spinda
Cacnea
Cacturne
Zangoose
Seviper
Castform
Kecleon
Shuppet
Banette
Tropius
Chimecho
Clamperl
Huntail
Gorebyss
Luvdisc
Deoxys
Turtwig
Grotle
Torterra
Chimchar
Monferno
Infernape
Piplup
Prinplup
Empoleon
Starly
Staravia
Staraptor
Bidoof
Bibarel
Kricketot
Kricketune
Cranidos
Rampardos
Shieldon
Bastiodon
Burmy
Wormadam
Mothim
Pachirisu
Buizel
Floatzel
Ambipom
Mismagius
Honchkrow
Glameow
Purugly
Chingling
Chatot
Carnivine
Finneon
Lumineon
Yanmega
Gliscor
Probopass
Phione
Manaphy
Darkrai
Shaymin
Arceus
Snivy
Servine
Serperior
Tepig
Pignite
Emboar
Oshawott
Dewott
Samurott
Patrat
Watchog
Pansage
Simisage
Pansear
Simisear
Panpour
Simipour
Blitzle
Zebstrika
Sewaddle
Swadloon
Leavanny
Ducklett
Swanna
Deerling
Sawsbuck
Alomomola
Tynamo
Eelektrik
Eelektross
Meloetta
Chespin
Quilladin
Chesnaught
Fennekin
Braixen
Delphox
Froakie
Frogadier
Greninja
Scatterbug
Spewpa
Vivillon
Litleo
Pyroar
Flabébé
Floette
Florges
Skiddo
Gogoat
Furfrou
Hoopa
Pikipek
Trumbeak
Toucannon
Yungoos
Gumshoos
Crabrawler
Crabominable
Oricorio
Minior
Komala
Bruxish
All Pokemon introduced in PLA and Gen 9+.
Source: Everything other than Gen 1 Pokemon listed here.
