The Gnome Extensions Website was for me a Problem, too in the Past. Did You tried it with the Extensions Manager? With that You can manage and install gnome Extensions. It is available in the Gnome Software Store:
Your native host connector does not support the following APIs: v6. You should probably update the Native Host Connector or install any missing extensions or APIs. See the documentation for instructions on installing the connector.
You typed there in the Search Bar ''Gnome Extension''. That is not neccessary because this whole Thing shows Gnome Extensions. You type there the Extension that You want or You search.
If You don't know what you really search You could search on the Website (You can free search there) and when You have found what You are looking for take the Name and put it in the Serach Bar from the Extension Manager.
To make it clear with in Example: Let's say you want a more customizable Task Bar; ther would be Dash To Panel a good Thing. So, You open the Extension Manager and go to the Browse Tab (in german: suchen) and type there ''Dash To Panel'':
For the Start Menu I can name You Arcmenu. But when You want the whole Zorin-own Extensions back I would suggest to use the Synaptic Package Manager. You can install it simply with the Terminal Command sudo apt install synaptic
When You have installed it, open it and search for gnome-shell-extension-zorin and then You should get a List with the Zorin-own Extensions that You can install.
If You want to begin from the Start a fresh (Re-)Installation of Zorin is an Option, too.