[davidf560]

DirectShow (the API for video capture and other things in Windows) has long had the concept of a filter which can be plugged into the video pipeline[0]. I'm not certain if that's the reason that the term is commonly used for effects such as the one discussed here, but this "cat filter" certainly might have been implemented as a DirectShow filter, so it's very plausible the terminology comes from that.

I'm pretty sure apps like this were called filters long before Instagram even existed.

[0] https://docs.microsoft.com/en-us/windows/win32/directshow/di...