Its very much still needed and heavily utilised in the enterprise world. Volume size is usually the lowest priority when it comes to arrays, redundancy and IOPS (the amount of concurrent transactions to the storage) is typically the priority. The exception here would be backup and archive storage, where IOPS is less important and volume size is more important.

As far as replacing sectors goes, I've never heard of this and I might just be ignorant on the subject but as far as I know you can't "replace" a bad sector. Only mark it as bad and not use it, and whatever was there before is gone. This has existed since HDD days. This is also why we use RAID - parity across disks to protect data.

Generally production storage will be in RAID-10, and backup/archive storage in RAID-6 or in some cases RAID-60 but I'm personally not a fan.

You also would consider how many disks are in the volume because there is a sweet spot. Too many disks = higher likelihood of total array failure due to simultaneous disk failures and more data loss in the event it does, but too few disks and you won't have good redundancy, capacity or performance either (depending on RAID level).

The biggest change I see in RAID these days is moving away from hardware RAID cards and into software-based solutions like Microsoft Storage Spaces, md, ZFS and similar. These all have their own way of doing things and some can even synchronise the data with other hosts.

Hope this helps!