Try not to over-think this rule change, guys, and please consider these two points :
(1) If there is a tack-on foul on the last play of a period, the opponents can decline the penalty and the period has ended.
(2) If there is a tack-on foul on the last play of a period, the opponents can accept the penalty and have an untimed down. UNLESS...
FRIDAY TRIVIA TESTER..WHAT WOULD BE A PLAY WHERE THEY COULD ACCEPT THE PENALTY BUT NOT GET AN UNTIMED DOWN?
(HINT: FOUL BY K THAT DIDN'T QUALIFY FOR TACK-ON)
After thinking about it I realize that some of my previous posts are not entirely correct and therefore, I believe beagle's plays answer the trivia question.
After a safety, R is NOT the next to put the ball in play. K puts the ball in play by a free kick. Therefore, if there is a scrimmage kick down where the end result is a safety, any foul by K is NOT eligible for the tack on option.
Of course, if the safety is due to the foul itself, then 3-3-4 kicks in, and there's no untimed down.
EDIT -- To put a specific situation in, K holds in the end zone during a punt as time expires. Accepting the penalty results in a safety, no period extension.