Can someone explain to me what the "cleanse levels" feature is? I know it makes an up-to-date version of the levels, but when does one use it? How often should it be done? After every major NL release? Should players be doing it too, not just level designers?
Basically, it just makes the player load and re-save every level. The new copy of the level file will be completely up to date format wise, all meaningless lines stripped, etc. Why exactly this works (rather than just spitting out an identical file) is a bit technical, but in short - if there's been a level format change that other tools no longer understand, but the player still does, this can get those levels into the current format so that other tools (like the editor) can use them again.
My current intent is that all versions up to (but
not including) the final version, will retain all backwards compatibility
that exists now, if possible to do so. So going forward, it shouldn't be necessary for any content that already works on the current version. But it's there just in case, as well as for content that hasn't been brought up to date yet.