While making a map, I noticed that I can place note while slider is playing. Is note mid slider verifiable?

Nope.

Only mania of all four is allowed to do that without it being unrankable.

can still make for some really cool unranked maps though (or even loved if you're both lucky and make a good map)

heres a couple of these kinds of maps by Voltaeyx, usually called 2B

beatmapsets/756794#osu/1605148
beatmapsets/790928#osu/1659472

also check out this by dsco beatmapsets/1894682