>>/9063/
I don't know about endchan, but on 8 there was a checkbox in the top thread settings area.

Eventually became a box you could enter a number, so bumplock after N number of posts.

Both are the same -- should be set to zero. Otherwise, inactive threads drift as active ones retain.

I would imagine a control+f "bump" will give you the appropriate setting, but I'm not familiar with endchan's moderation settings.