Currently you can download the loops and instruments again, but the next upgrade might change the set of available instruments and loops. I recommend strongly to keep a copy of the instruments and loops on an external volume.
After each major GarageBand upgrade I have been very glad, that I kept the older loops and instruments, or my older projects would be showing missing instruments. The earlier GarageBand versions, (before GarageBand 10) had much more software instrument loops, for example. I prefer the software instrument loops to the newer real instrument loops. They are much more flexible and do need less storage. If you created projects with the current sounds, keep an archive copy in a folder somewhere. Do not rely on Time Machine to archive older software. Time Machine will remove older backups, if it is running out of storage. IT is great to recover older, missing files, but not a full archive of very old files. The history is limited.