I've seen several other similar questions on Stack Overflow, but they're fairly old / potentially outdated so I'm wondering if there's any new packages or methods for making SOAP requests in Go. Thanks!
I think there are none.
To begin with, there are two documents describing soap—the so-called "SOAP Note" which is officially not a standard but is widely referred to as "SOAP 1.0", and another one, which is the standard and is commonly referred to as SOAP 1.1. They are not compatible even though they look superficially the same.
On the other hand, SOAP itself—as XML encoding of what the client sends, and what the server responds with—is simple, and in fact crafting a simple SOAP wrapper (to make client calls) and unwrapper (to decapsulate server's responses) is simple.
I'd like to stress this: both SOAP standards do not actually deal with HTTP as SOAP is explicitly defined to be transport-agnostic (it hence suffers from the same problem the ReST paradigm does: everyone thinks it's about HTTP), and the Go's standard library features excellent support for HTTP, so the SOAP encoding/decoding layer appears to be rather thin.
With these considerations, I, personally, did not even bother to start depending on a 3rd-party package for SOAP in each of the cases I needed to call something over SOAP and wrote the necessary code myself; I've just checked that the last time I needed to do SOAP 1.2 calls, the SOAP encoder/decoder ended up being 171 lines of Go code (excluding tests).
So I'd say just roll your own.
Note though that it's all that simple only if your server does not require something awry to use—for instance that dreaded WS-Security extension which requires special formatting of XML to be sent, calculating cryptographic hashes over it, digitally signing them and all such stuff; there's just nothing to handle this in Go as of yet.
The same applies so WSDL specs or XSD schemas. IIUC, there presently is no tools of sufficient quality which would generate type hierarchies and SOAP endpoint client stubs for you given a WSDL document or an XSD schema.