Does this replace vanilla behavior?

No. It adds two shortcut tools and only affects items you select with the area cleaner.

Is this safe for existing saves?

Yes. It stores only per-player filter settings and does not alter recipes, technologies, or entity prototypes.

Does this affect achievements?

Like any mod, using it in a save will use the modded-achievement rules for that save.

Does this require Space Age?

No. It is built for Factorio 2.1 and works without Space Age.

Does it work with other mods?

It should work with normal transport belts, underground belts, splitters, loaders, loader-1x1 entities, inserters, and dropped items. Modded belt-like entities need to expose normal transport lines.