Ascending integer pyramid
Jelly,  13 12 10  6 bytes
-4 thanks to Dennis, yes FOUR! (use of group indices and Cartesian product)
⁵pḅ1ĠU

Uses 1-indexing and the list option for output.
Try it online! (The footer formats the output in Python representation)
...or see a 0-indexed, formatted version.
How?
⁵pḅ1ĠU - Main link: no arguments
⁵      - literal 10
 p     - Cartesian product (with the leading constant of 10 and implicit ranges)
       -       = [[1,1],[1,2],[1,3],...,[10,8],[10,9],[10,10]]
  ḅ1   - to base one (proxy for sum each without the monad)
       -       = [2,3,4,5,6,7,8,9,10,11,3,4,5,6,7,8,9,10,11,12,4,...,18,19,20]
    Ġ  - group indices by value
       -       = [[1],[2,11],[3,12,21],...,[90,99],[100]]
     U - upend = [[1],[11,2],[21,12,3],...,[99,90],[100]]