Need suggestions on youtube exchange

So i hope im posting in correct area

Ok so i used a youtube sub exchange site. And it worked for my friend channel and it is permanent and i too did for my channel but suddenly subs vanished to half and yes its upto 80 in subscribers panel but actual count is diff.

So i did some research and seems youtube deletes some subs. If so what is correct pattern to use the sub exchange sites ? i did find some threads but they old and wanna know some updated method.

Thanks in advance.

Youtube never took my subs away, probably a lot of account in this sites were banned.