I was with Tesco from Jan 2007 until Mar 2020 with the yearly delivery contract in place. Even though I had 8 months left on the then-current contract in Mar 2020, they stopped offering me delivery slots. I checked at a few points throughout the rest of 2020, even in between the lockdowns, but to no avail. That was 8 months of delivery contract wasted.
Meanwhile, I signed up to Asda, totally new customer i.e. haven't used them before, and I got given a delivery slot every week from Apr 2020 for the reminder of 2020, without fail. I'm still with Asda now in Dec 2021.
It seems to me that Asda has handled the pandemic better than Tesco and it's not relevant if I'm a new or old customer at either of those places.