I think the core issue isn't that it's a new skillset - you don't even really need to slide that well, you just need to clearly be "giving yourself up" which is most obvious with a slide down to the ground. The problem is that QBs are pushing their rushing attempt to the absolute maximum and then trying to slide right before someone is in tackle distance which is just foolish. A slide needs to be initiated well before someone is actually in range to hit you.
IMO they should treat a slide as a legitimate slide only if no defender is within three steps of the player sliding. If there is a player within that proximity at the time of the slide, then the slide is void from a contact standpoint and any hit the player incurs as a result will not be flagged.
This would stop "late" slides as the player would know they need to slide well in advance or they're gonna get hit regardless, and it stops the defender from being faked out by a fake slide if they are within hitting distance.
The "faking going out of bounds" thing is different and just needs to be unsportsmanlike conduct.