It is an issue in sequential code because as you say, that's just how slices work. But if you're always using the same variable you'll never encounter it because that slice can't change between you reading that variable and writing to it.

Once concurrency is introduced you can now read from the same variable, but another goroutine may have written to the same slice in the meantime. That's why you must protect the read and writes and synchronise them.

It's fundamentally just a race condition issue with unprotected reads. But people often overlook it in the case of slices because they think they're just taking a reference to the slice, which is safe to do concurrently IF slices were reference types. But they're not, they are copied.