There seems to be a common misconception throughout this thread that ACARS is sending back position reports constantly. It doesn't.
Perhaps not constantly but ACARS on some planes it definitely sends out a periodic position report.
Here is hobbyist software that will plot those ACARS position reports:
PlanePlotter plots aircraft positions, altitudes and times decoded from the message traffic that it receives. These include embedded position reports, AMDAR reports and ADS reports contained in ACARS messages,
PlanePlotter from COAA
Also, see:
Can ACARS send postion, altitude and heading information automatically?
ACARS itself can not, but other systems like the Flight Management Systems (FMS) or ADS-C on board the aircraft can use ACARS to send reports like this automatically, and in some cases they do.
avionics - What data does ACARS send back to base? Can it be used to track a plane? - Aviation Stack Exchange
Looking at VHF ACARS logs, some carriers seem to still spit out a periodic position on ACARS, others don't, perhaps like the engine data, depending on the avionics, service providers and level of service purchased.
Commercial versions of ADS-B tracking software may have already rendered this ACARS position reporting obsolete but there is a lot of legacy stuff at a money losing national carrier from my past experience.