Firmware updates are more important when used as a router where the device is exposed to the Internet so any software bug could leave it open to exploits.
When its used as a bridge modem its only relevant if there is a modem problem impacting sync, which as DSL is unlikely to get much development now most countries are moving to fibre, its unlikely the modem firmware is getting much if any work on it.
Its already had years of development that has gotten it to such a well optimised stable modem.
