Yes custom fields and project automations will remain.
Portfolios, goals and the features related to those are the bulk of what you’ll lose when downgrading.
I’d say those are essential to upgrade Asana from a fancy to-do list to a work management system (especially when combined with status updates done well) but that is of course for you to decide.
If there are any features you are specifically curious about, the help center articles mention all plans features are available in, if it’s not available in the free tier.
PS: You can find an overview of features per plan at Asana Pricing | Personal, Starter, Advanced, & Enterprise plans • Asana