In a small sense I have done sub4sub, though it isn't always blatant. for example, I make Far cry 4 videos, I type 'far cry 4' in the search on youtube, filter for 'newest' , then I watch people's content, and after a while of supporting their channel and staying active, I would say ' hey! I just posted a vid this morning, would you mind taking a look?'
I do this less commonly as I have grown as a channel, but it is a good way to build a small following of dedicated subs to kickstart my channel. Now I get around 100 views on each video and 15-20 likes!
Hope this helped to give you a different perspective!