You can get the primary site via craft.app.sites.primarySite, but I'd still +1 this – needing to refer to the primary site in multi-site builds is pretty common, so having a global primarySite variable/shortcut would be a nice convenience feature.

@jamesmacwhite That siteUrl() example is a perfect use case for named arguments in Twig 😉:

{{ siteUrl(siteId=1) }}

Or, if you want to avoid hard coding the ID:

{{ siteUrl(siteId=craft.app.sites.primarySite.id) }}

It would be cool to be able to do things like this though:

{{ siteUrl('login', siteId=primarySite.id) }}

@mmikkel Thanks! I've learnt something new with named arguments today with Twig, to make siteUrl usage a bit cleaner for sure!