Is this really considered unusual? USUM only introduced some new legendaries and the dusk form for Lycanroc right? Would've been legitimately surprising had we gotten entirely new evolutionary lines instead.
And do neither Keldeo nor Hoopa count as mid-gen? I always thought those were officially revealed and distributed post release, but could be wrong as I don't pay much attention to legendaries in general
Keldeo and Hoopa were revealed post-release but were in the original games.
Gen 7 was the first where they added ones to games within the generation that weren't in it, though you could argue Gen 6 was the first due to Mega Evolutions, or Gen 3 if you factor in forms, though Gen 4 is the more prominent one for that.
Here's a timeline
Gen 3
2002: Ruby & Sapphire Released
2004: FireRed & LeafGreen released. Adds Attack Forme & Defense Forme Deoxys
2004: Emerald released. Adds Speed Forme Deoxys
Gen 4
2006: Diamond & Pearl released
2008: Platinum released. Adds Rotom's 5 forms, Giratina Origin Forme & Shaymin Sky Forme
2009: HeartGold & SoulSilver released. Adds Spiky-eared Pichu
Gen 5
2010: Black & White released
2012: Black 2 & White 2 released. Adds Black Kyurem, White Kyurem, Keldeo Resolute Form and the Therian Formes of Tornadus, Thundurus and Landorus
Gen 6
2013: X & Y released
2014: Omega Ruby & Alpha Sapphire Released. Adds new form of Hoopa Unbound. Adds 18 new Mega Evolutions. Adds 2 Primal Reversions
Gen 7
2016: Sun & Moon released
2017: Ultra Sun & Ultra Moon released. Adds 5 numbered Pokémon (Poipole, Naganadel, Stakataka, Blacephalon, Zeraora) and Lycanroc Dusk Form
2018: Let's Go, Pikachu! & Eevee! released. Adds 2 numbered Pokémon (Meltan, Melmetal)
Really it was inevitable. I fully expect the same in Gen 8. Heck, I'm hoping they finally leave Mythicals out and add them with patches later on.