Dataminers have been poking around in Splatoon 2, and they've made a rather interesting discovery. A pair of voice chat icons have been discovered, which simply show voice chat being on/off. Splatoon 2 currently supports in-game voice chat through LAN mode, but regular online play requires you to use the Switch Online mobile app.
The LAN voice chat was tested after this discovery, and neither of the icons appear while chatting. This means that whatever the icons are for is yet to be implemented. It could be part of an upcoming feature, or was an element that was once being considered, but has since been dropped.
Also worth pointing out, a parameter called “BitRateMeasureRate” was added to the game’s versus config on June 13th, as well as the main game config file in Version 4.0. This pertains to the bit-rate of voice chat.