Hi -
AFAICT it seems that you're correct, the macOS version of GB includes a Tempo Track where you can set up Tempo changes [a varying Tempo map], but this feature apparently is not included in the iOS version. I only have the Mac version of GB, but a search for 'tempo map in GarageBand iOS ' turned up a suggestion for a workaround -- however this seems to be basically to create a separate Project at a different Tempo, bounce out (Share) an audio file at that different tempo, and then import it into the Project at the original Tempo. I guess if you don't enable Flex on the track with the imported audio that imported audio file will play at the different Tempo, even though it won't line up with the original Tempo in the ruler. Seems kind of clunky -- especially if there are a lot of Tempo changes -- but that's all I could find..
