I, too, remember a few years ago when the same baggage allowance applied to all flights in a oneworld award. This was especially useful if the itinerary included the Americas as, if I remember correctly, you'd have two pieces to begin with, even before elite benefits. These days, though, I think the baggage allowance varies by carrier and flight.
The baggage rules haven't changed for years.
Where the ultimate/furthest ticketed destination is in the US or Canada (commonly the case for awards starting from PER), you get the same allowance across the entire ticket as the US DoT/CA CTA exception rule applies. The selecting carrier in this case is the first marketing carrier. The selecting carrier can apply their own allowances, or defer to the first most significant carrier. These rules are filed with ATPCO and not readily publicly available info.
A commercial fare example to illustrate this point on an identical routing with different feeder carriers...
QR allows the use of either AC or WS as the Canadian domestic feeder on their fares to/from Canada.
YYZ WS YUL QR DOH QR SYD in Y - WS is the selecting (first marketing) carrier. WS has selected the most significant carrier's baggage policy should apply, which is QR so QR's 2 piece policy applies.
YYZ AC YUL QR DOH QR SYD in Y - AC is the selecting (first marketing) carrier. AC has selected that their own policy should apply, which is 1 piece.
In such a case I would ticket a pax on WS for the domestic feeder so that they receive 2 pieces as their allowance.
Another example but with codeshares...
QR allows the use of either QR codeshares operated by AA or AA primes on their fares to/from the US for the domestic feeder.
DEN QR*AA PHL QR DOH QR SYD in Y - QR is the selecting (first marketing) carrier. QR has selected that their own policy should apply, which is 2 pieces.
DEN AA PHL QR DOH QR SYD in Y - AA is the selecting (first marketing) carrier. AA has selected that their own policy should apply, which is 1 piece.
Where the ultimate/furthest ticketed destination isn't in the US or Canada (commonly the case for awards starting from the east coast), then you get all the varied allowances (IATA resolution 302) as the US DoT/CA CTA exception doesn't apply.
Key east coast examples...
Where an itinerary starts from SYD and contains both LHR and BOS... the varied allowances (IATA resolution 302) apply as SYD-LHR is further than SYD-BOS.
Where an itinerary starts from MEL and contains both LHR and BOS... the same allowance (US DoT/CA CTA exception) applies throughout as MEL-BOS is further than MEL-LHR.