This Yoda," the Emperor mused. "Lives he still?"
And he doesn’t make a very big deal out of it when he finds out he’s dead, other than the fact that there’s no more Jedi to train Luke.

From this dialogue I’d say it’s heavily implied that Palpatine has never actually met Yoda. He seems to only have heard of him, which I guess makes sense as he trained Obi-Wan and possibly others, and he presumably had a reputation.

Then, how would you interpret the fact that the Emperor knew the speech pattern of Yoda? By what he read on Lukes mind? For me, it would actually implied that they met, at least once