How to track when all team players die (mc 1.16)

First, take a look at the answer to the question "Use tellraw to display a player death message about the player who died", in order to setup a scoreboard which tracks deaths. (The goal here is very similar to yours, so we can adapt this answer to suit your needs.) The scoreboard is set up the same way:

 /scoreboard players set @a hasDied 0
 /scoreboard objectives add hasDied deathCount

In order for all players to have died, that means the lowest score on the scoreboard is 1 or higher. So you would want to test for score = 0. Try a command block with:

/testfor @a[score_hasDied_min=0,score_hasDied=0]

This works because without a qualifier the test checks for score <= 0, and score <= 0 && score >= 0 EQUIV score = 0. See this answer for more clarification.

You can then chain any commands you would like to execute if the test fails. To check for a failed test, you need to invert the result of the command block. See this answer on how to do this.