My first exposure to Metal Slug was actually in regular emulators, and I never used the scanline filters, so now when I use the scanline filters in Metal Slug they feel..."wrong". In my mind, Metal Slug is supposed to have really sharp, chunky pixels.
Not my case; I'm old enough to play it at mid-late 90's in both bars and arcade rooms.
And that's the problem with current pixel art artists: they have no idea of what actual pixel art looked like. Hint: look at Garou with at least scanlines (or maybe a bilinear filter) enabled. That's what's Garou almost meant too look in CRT, far closer than raw pixel art.