What can be Blocked by the Hylian and Deku Shields?

Solution 1:

In short, most projectiles can be blocked by Link and Young Link's shields.

This wiki about Link's shield states:

To be considered a projectile for the purposes of the shield, a hitbox must be reflectable and not unblockable.

There is no distinction made between Link and Young Link (or Toon Link for that matter). However, Link's shield in Smash 64 does not block anything.

This wiki lists all of the Smash series projectiles and whether or not they are reflectable (and in which Smash, if different).