Hello!
At this point in time there seems to be no end of life date for Application Gateway V1 according to https://learn.microsoft.com/en-us/azure/application-gateway/application-gateway-faq ,however this might also be an oversite since its a dependency issue. I remember another article about it that I could not find that stated that customer should be informed well in time if this where to change, to make migration possible.
"Yes. The Application Gateway v1 SKU will continue to be supported. However, it is strongly recommended to move to v2 to take advantage of the feature updates in that SKU. For more information on the differences between v1 and v2 features, see Autoscaling and Zone-redundant Application Gateway v2. You can manually migrate Application Gateway v1 sku deployments to v2 by following our v1-v2 migration document."
Hope this helps!
Kind Regard Timmy