There are far too many different repositories with different configurations to give an answer valid for everything.
However,
> But are there any (download) limits or throttling mechanism going on if the repositories are "misused/abused/attacked" like that? Is it possible to get an IP-Ban?
Some repositories do enforce limits, and it is possible to get an IP ban on them.
Basically, if you use repositories in a responsible fashion, you shouldn’t run into problems. If you start trying to overload them, you can expect to run into problems at some point or other, including IP bans which may have a bigger impact than you expect (they might affect more than the specific repository which triggered them).