As you can see in this example: 

It's caused by the size 8.5 is out of stock in width, so the red line appeared to distinguish products out of stock and in stock

But If you want to remove it, you can go to this link: 

https://vandal2.staging.screencommerce.com/static/version1628006061/frontend/Mgs/unerosp/en_GB/Magento_Swatches/css/swatches.css

Then, remove background ( highlighted in red) as you can see in our snapshot to disappear the red line as you want: 

And you still can reback If it's necessary with you in the future. However, we think it will be better If you show the red line to have a clear distinction.