Hello FroggyC,
I’m unsure when this changed however there have been a few topics such as this Issues when using slow mode on a topic forever stating their opinions on allowing slow mode forever.
Topic slow mode as Jeff pointed out below is not designed to be used permanently it seems non sensical to bring it back too since a topic won’t last forever since it will auto close once it reaches a certain number of replies. Perhaps you can select a custom date and time like this? Maybe don’t make the date as ridiculous as mine as it was just for demonstration 
