I still find the rushed/not rushed debate with this situation a bit odd. It's reasonable to debate whether NHL or AHL is a better place for him right now, but I don't think the "rushed" label is really appropriate here. To me, "rushed" does not mean "is playing in the NHL as D+1 or D+2 prospect but isn't putting up pro-rated 50+ point production", it means "playing in the NHL too early where glaring technical skill gaps or physical limitations force a player to drastically strip down their game to barely survive".
The latter doesn't describe Slafkovsky at all, he's playing his game and using his physical tools to make plays, learnining, progressing, and taking risks on the ice. This isn't rookie Victor Mete going off the glass every play because he has to play the most basic game imaginable just to keep his head above water, he's learning and figuring out where the limits are at the NHL level. If he keeps getting smoked on big hits like this then sure I think it would make sense to send him to the AHL to acclimatize to small ice in a slower paced league, but his overall play and production don't really concern me in terms of whether or not he's being "rushed".
Once again, this is exactly how European clubs develop young players. It's just rare in the NHL because most 18 year olds simply can't handle the physicality of the NHL, while European leagues play on bigger ice where physicality isn't as big of a factor. We have a unique prospect who can handle NHL physicality, so he's playing in the NHL.