To be honest, I can kinda get behind the reason for doing it, though I think it's a stupid choice.
Slanns have been... difficult to balance. They are tough as nails, have inherent ward saves, and a whole slew of disciplines specifically designed to make them better casters. Their 8th edition rules are far weaker than their, frankly, overpowered rules back in 7th. They did a fairly good explanation for why they have grown weaker, but it only gets them so far - GW doesn't seem to like how Slanns currently are, being these super powerful, and difficult to kill, wizards. One could just say "Nerf them more, then", or whatever, but in all honesty, would you prefer:
1. Slanns getting removed
2. Slanns getting nerfed to the point of being literally just a standard level 4 wizard, with no ward save, T3, etc.
Honestly? I'd rather take 1, and get a Skink High Priest instead. It would suck to lose our magicical edge, but I'd much rather lose it by losing the Slanns entirely, than see our Slanns get nerfed that hard.
My main issue with their string of nerfs to them, is that I do not see the issue with Lizardmen being among the best casters. High Elves have super powerful wizards, as does Chaos, and there doesn't seem to be any issue with them being this strong. Why should it be an issue that Lizardmen are good at magic? Especially considering we aren't good at much else - In melee, we are outdone by most armies, in range, all we have are short ranged skinks, who are primarily good because they are both pretty good at shooting, and have some really good movement thanks to quick to fire + skirmishing.
I have to admit, though, I AM looking forward to what will become of our Lizzies. I am confident that, whatever might come, we'll still come out okay on the other side. If we are lucky, and considering the recent addition of Marks of Chaos on Beastmen, we might even see some actual customizations for our Saurus, and maybe some better internal balancing between them. I would certainly enjoy my Saurus becoming the actual "born warrior" type of guy, and if we go skirmish-level warhammer, having a unit of lizardmen free-running through a forest towards an enemy, expertly manouvering through the woods like it wasn't even there (being used to the dangerous jungles of Lustria), and slamming directly into the enemy lines, would be absolutely fantastic.