I just can’t find or recall how I did this earlier…
The category setting Auto-close topic after:: how do I disable it? Now it allows change time ranges only.
Category settings > Moderation > Set the below to blank or 0.
Once done, you can save from there.
That was it. Thanks.
Not too intuitive way, though — or I’m too limited 
You are not alone. It was moved recently to the ‘Moderation’ category settings tab; I had my time trying to find it on the ‘Settings’ one.