I guess where on Imgflip, ie which stream you are asking about may matter. Owners of user created streams can increase what is considered NSFW for their streams, if they want to.
Sometimes Stream Moderators for user created streams apply their own standards as well. How a user created stream deals with all that is well, up to the stream.
Imgflip does not consider "shit" or "bitch" to be inherently NSFW. By default "shit" or "bitch" creations in the fun stream are in fact SFW.
imgflip.com/tag/shit
imgflip.com/tag/bitch
In general, any images in Imgflip owned streams with "shit" or "bitch" that are NSFW were either set that way by the image owner during the creation/submission process or also have some other NSFW image or vulgar text content that is NSFW.