Ignoring the fact that Medica II as of Endwalker launch also shares 2s cast time with Medica I, your wording in this post particularly implies that 'any spells with [GCD =< Cast]-cast time cannot be slidecasted', when---as couple other have mentioned, you still can.

But then you doubled back later on:

Quote Originally Posted by Renathras View Post
You have about 0.5 sec on a 2.5 sec cast. This is highly latency dependent and also results from the communication between sever and client. Try to slidecast with a Slipstream. You can, but you're very likely going to clip your GCD to do so. If you don't care, that's fine, you can do that. But that's what I'm referring to. You can get some motion, but you're cutting into your next CD with longer casts.
So which is it now? I'm only pointing this out because you're giving off potential indication that you may have different--or worse, misconception of what a 'slidecasting' is.

And no, Slipstream is not even close on being clip-risky when their cast time is a tad shorter than its GCD. If any, only BLMs that will actually clip simply for existing because at least half of their spells have longer cast time than its own GCD, slidecasted or not.