Shopify $_GET variables via Liquid

get-vars.liquid

<!-- Add this at the top of your page template -->

{%- capture contentForQuerystring -%}{{ content_for_header }}{%- endcapture -%}
{%- assign pageUrl = contentForQuerystring | split:'"pageurl":"' | last | split:'"' | first | split:'.myshopify.com' | last |
   replace:'\/','/' | 
   replace:'%20',' ' | 
   replace:'\u0026','&'
-%}
{%- unless pageUrl contains "?" -%}{% break %}{%- endunless -%}
{%- assign pageQuerystring = pageUrl | split:'?' | last -%}
{%- assign tag = pageQuerystring | split:'=' | last -%}
{%- assign query_tags = tag | split: "," -%}

<!-- Try adding ?tag=blah,blah2,blah3" at the end of your page's URL. -->
<!-- // Example: -->
{{ query_tags | json }}

For now it's probably just best to stick to good ol' JavaScript:

const params = new URLSearchParams(window.location.search);
const value = params.get('email'); // ?email=test for example...
console.log(value);