These are songs from the Billboard Hot 100 Year-End list of 2009 that sound more 2000s than 2010s:
Poker Face - This song sounds primitive compared to Electropop tracks of the early 2010s
Just Dance - Same as above
Love Story - Power Chords in choruses were very common in the 00s.
I'm Yours - This song sounds the same as those Acoustic songs like Bubbly and You're Beautiful
Single Ladies - Typical late 00s R&B production
Gives You Hell - A Power Pop song
You Belong With Me - Same as Love Story
You Found Me - Doesn't sound very different from their 2006 stuff
Use Somebody - There are elements of Post-Grunge in this but the vocal style leans 2010s
Knock You Down - Again, typical late 00s R&B production
Blame It - You can hear Crunk synths in this song
Kiss Me Thru The Phone - This is a Pop-rap song with Snap production
The Climb - Typical Pop rock ballad at the time
My Life Would Suck Without You - Sounds almost like her 2004 material
Halo - An R&B Power Ballad
Hot N Cold - Katy Perry's music wasn't Electropop at the time, she leaned Pop rock in 2008-09
Second Chance - A Post-Grunge song
Circus - The production was clearly inspired by Timbaland
Let It Rock - The production blends Southern Hip Hop and Rock
LoveGame - Do I need to say anything about this one?
Waking Up In Vegas - Same as Hot N Cold