Double tap to zoom stopped working after upgrading to iOS 13
It appears that you now have to triple-tap to zoom in on things like paragraphs in Safari.
This answer doesn't enable double tap zoom feature.
Double tap is being used to select a word in text on iOS13, so it might not work correctly when used not for the same.
Page zoom can be done using the AA looking icon. The settings for individual websites can be edited in Safari Settings.
https://ios.gadgethacks.com/how-to/make-text-bigger-smaller-ios-13s-safari-without-affecting-rest-webpage-0200096/