In the first issue of Spider-verse, when all the spider-people first meet we see Mayday take her mask off and say "You know me" to 616 Peter, to which he replies "Mayday?!" implying that they've met before. I never read any of MCU2 so I tried looking this up to see if this were actually true but couldn't find anything.

Did Slott just make that up for the sake of the story or have they actually meet before, and if so what issue?

Mayday did indeed cross over into the 616 universe in two issues of her own book. In Peter's timeline it was set around ASM#25 ("Captured By J.Jonah Jameson"), along the way she also met Aunt May and Mary Jane in the same story. She also encouraged Aunt May to continue to try and pair Peter and MJ together.

While this was the first instance of father and daughter meeting, other versions of Mayday have went back in time and met Peter too, like in the UK Spidey comics