I downloaded all available sounds in the sound library in Garageband 10.3.4. Some instruments that I've seen used in older YouTube videos are missing from my library. Melodica, some skank reggae guitar, etc. Am I missing something or did Apple remove a bunch of instruments? I only see 5 guitars for example.

Hi InFocus - I assume you're looking in the Library pane at the right.. In the root level of the Library pane maybe try looking under Legacy -- older instrument Presets might be found there [i.e. Jam Packs] But keep in mind, if someone has Logic on their system some Logic sounds *might* show up in the GB instrument menus as well, which might account for instruments you've seen in GB elsewhere that you're not seeing on your system. [FWIW I don't see a Melodica in GB either even though I have a Melodica .exs Instrument in my Logic Sampler Instruments folder -- AFAICT GB doesn't show .exs files, only Patches]

What do you mean the library pane at the right and the root level? The library section in my GB is on the left.

Yes, sorry, mental typo -- the Library pane on the left.. :-) The root level would be the first (left-most) panel in the Library pane.