Preclear has NEVER been necessary. Ever.
Originally, many many years ago, unraid wouldn't fully start and allow you to use it while a new drive was being cleared in preparation to participate in the parity calculation. That created a need for the ability to clear the drive before you added it, and pre-clear was born. It matured into a drive testing tool and continues in that role today.
Now, when you add a drive to a parity protected array, Unraid starts fully and allows you to use all the normal array functions while the newly added drive is cleared in the background. When it's done clearing, you will be given the option to format it and start using it to store data. Pre-clear isn't necessary now, and it wasn't necessary before, just a convenience.
However... because unraid requires the full rated capacity of ALL drives, even empty ones, to be perfectly readable to accurately reconstruct a failed drive, it is prudent to test drives before trusting them in the array. This can be accomplished with the drive manufacturers tools, third party drive testing utilities, or preclear. It's just one option, a very thorough option, to test drives.
If a drive is precleared successfully, then as long as it remains in that state with no changes, format, partition or otherwise, then when you add the drive to the parity protected array unraid detects the special preclear signature and trusts that the drive is clear and immediately offers to format it instead of clearing it. However, the preclear utility takes WAY longer to do its job than the built in clear which doesn't do testing and verification does, so the fastest option is indeed just to plug in the new drive and add it to the array.