This isn’t particularly recent news but it hasn’t been reported on much for how significant it is. The TL:DR is that many 4G phones use 3G for calls or don’t support VoLTE with Australian telcos. The shutdown will leave a significant number of Australians with phones that cannot make phone calls, forcing them to buy a new one.
It’s not about turning off 4G, but rather that many 4G phones use 3G for calls. Those that don’t need to have VoLTE enabled, which can also depend on compatibility with the carrier and whether the phone is a locally distributed variant or imported.
Many 4G phones will become useless as phones if/when 3G disappears next year.