Exposed Liquid Variables and Attributes

Is there a list of Liquid Objects and their attributes we can use for themes?

No. I believe the most that has ever been said is that all variables are used in the built-in, Plain theme. Of course, the variables mean different things, depending on the context of the form for which they are invoked. Recipient, for example, can be a customer, supplier, payer, payee, employee, etc., as needed.