Everything on the site will say updated the date you look at it. If you check peoples' membership pages and garden listings, they will state "updated" the date you look at them, as will all nursery listings. It appears to be the default setting for the database. I didn't program it but I do see it regularly researching garden, membership and nursery listings.
If any nursery has the word Archive after its name or on its page, it means it is no longer in business. The plant list is there just to show what they used to have available, for research purposes.