After a little bit of digging, I found an article which inform us:
As we all know, video formats, such as MKV, MP4 and AVI, can contain several subtitle streams in different formats or languages in one file. Technically, subtitles are divided into two types: hard subtitle and soft subtitle in video files. Hard subtitle is a part of the video images, and you cannot disable or hide it when playing the video file. Soft subtitle is an independent stream, and you can turn on or turn off it when playing.
If the subtitle can be removed, here is a method on how to remove subtitle from MKV, MP4, AVI, etc with ease.