There's been a ton of stuff since 1.8. This will obviously be a brief summary but here's every major update since:
1.9 - Combat update. Overhauled End.
1.10 - Frostburn Update. Some new nether blocks and new snow mobs
1.11 - Exploration Update. Added woodland mansions, new hostile and friendly mobs, and a bunch of new useful items
1.12 - World of Color Update. Some quality of life stuff with color blocks I guess
1.13 - Aquatic Update. Overhauled everything rivers and oceans. New mobs
1.14 - Village and Pillage Update and Texture Update. Added village raids and overhauled a ton of textures
1.15 - Buzzy Bees. Added bees
1.16 - Nether Update. Complete overhaul of the nether
1.17 - Caves and Cliffs. Completely overhauled terrain generation
1.18 Caves and Cliffs part 2.
1.19 - Wild Update - Added new scary underground biome and mob
1.20 - Trials and Tales. Added archaeology and new mobs.
1.21 - Tricky Trials. Added trial chambers
The music one is because you don't double count the first octave C.
C D E F G A B C D E F G A B C is 15 notes.
A scale is really only 7 notes when you think about it when you don't count the octave. But in this case you count the first octave but not the second