Supermarket shelves are definitely less well stocked since Brexit. I sometimes use a local Aldi, and before Brexit it was rare for anything to be out of stock. Their stock replenishment was far better than competitors. In the last year or two, they've actually removed some shelving to disguise less volume and variety of products. Which supermarket would willingly do this - commercial suicide?
Anyway, certain products are now regularly out of stock. Eg. buying eggs there is now often hit or miss for all but the most expensive free range ones. There are gaps in the fruit and veg displays where there were none before Brexit. Getting bagged salads is not always possible and the choice of them is reduced - for example. From Jan 1st 2021 their lactose free milk disappeared from the shelves and reappeared some MONTHS later. The special weekly offers in the central aisle are now often not available when advertised in their magazine - this never happened before Brexit.
These are just some of the problems I can think of there since early 2021 - there are many others. There are definitely big supply and logistics problems for this retailer and others, and the shortages and reduced range are obvious to all who want to see.