Time is very much still broken down in to 6 second combat rounds in D&D 5e.
Who ever says they stopped doing this back in 2e is 120% dead wrong.
It's the last sentence of the first column under the heading 'Time' in the PHB. Page 181 for those playing the home game.
Every combat participant takes one turn within a 6 second round. It doesn't matter if there are 100 participants or 1 participant. The time span of a round in which all players take their turn is 6 seconds.