I sent my csv to Shopify support and they said I had a metafield which wasn’t accepted any more: Google Shopping / Custom Product
The metafield was set to Type Boolean and they said importing Boolean metafields is not possible.
I never had a problem before so this must be a recent change. All of the values for this metafield were set to false anyway so it wasn’t a useful metafield for me.
After removing that column from the csv it imported fine.