This is from Magento development docs
The selection of available variables depends on which template you use as a basis. The template-specific variables are contained in a comment at the top of each template on the file system. (For example, look at app/code/Magento/Customer/view/frontend/email/account_new.html.
You can read more https://devdocs.magento.com/guides/v2.0/frontend-dev-guide/templates/template-email.html
So you can rewrite <Magento_Email_module_dir>/view/frontend/email/footer.html and add your variable.
I found that you can also add customer variables in backend.
System>Custom Variables:
And one more information:
I found on Marketing => email templates=> and load Footer (Magento/luma)
and this template load address using:
<p class="address"> {{var store.getFormattedAddress()|raw}} </p>
Uptate: for Magento 2.3.4:
<p class="address"> {{var store.formatted_address|raw}} </p>
Ref:
https://github.com/magento/magento2/blob/2.3/app/design/frontend/Magento/luma/Magento_Email/email/footer.html
Migrating templates:
https://devdocs.magento.com/guides/v2.3/frontend-dev-guide/templates/template-email-migration.html
Or just switch to use legacy template in mail template table.