Not sure where those code snippets are from, but the only references I can find to upsert/get_fulldoc are in priv_constants.py and subdoc_t.py . It is mentioned here:
PYCBC-404, PYCBC-402: GET_COUNT and other subdoc additions
This adds the `get_count()` subdocument operation. It also adds support
for the `get_fulldoc()` and `upsert_fulldoc()` operations which can be
used to retrieve and modify the entire document, respectively."
According to the sdk-rfc mentioned in PCYCB-404 ( https://docs.google.com/document/d/1z3pJCPg77PZ8U8rFAyABuEhHSl68j1n42zP2RyflWZs/edit#heading=h.7ha864pzwf8m ):
"In the SDKs the proposed way to expose this is by allowing an empty path to the get() and upsert() sub-functions of LookupInBuilder. MutateInBuilder." - as Brett Lawson reiterated.
Seems pretty safe to remove as far as I can tell - will try tomorrow.