The IFAB has given the KNVB permission to experiment with a 4th substitution in the Dutch Cup tournement. It is only allowed in the extra time of the match.
Yes it did. The first one was in the semi-final game between Arsenal and Manchester City with City's Kelechi Iheanacho the man introduced.
The EFL Cup is also using it this year, and both Burnley and Leeds United used a fourth sub last night before their match went to ABBA kicks from the penalty mark.
MLS will use it this postseason and it was in play for the Confederations Cup and Gold Cup, as well. This isn't too newsy, as general permission to allow the 4th substitution was given at the last big IFAB meeting.
Remember the days when MLS had an extra "goalkeeper only" substitution rule? Teams could have a 4th sub if they were replacing their goalkeeper. It went away after Bob Bradley exploited it when he coached the Metrostars. Basically, he put a sub in for a field player who he moved to goalkeeper and had the prior goalkeeper move to a field player position. The next time the ball was dead. He notified the referee that the original goalkeeper and the substituted goalkeeper switch positions.