As per the Source's words : Google is all set to announce the Google Assistant for iOS users. Rumors saying that Announcement could come at Google's I/O conference.
Google Assistant For iOS Functionalities:
The app would likely feature a blend of the "chat" style functionality in the Google Allo version of Assistant and the voice-controlled version found on Android, but again, details are unconfirmed.
For the starting period, Google assistant will be only available for US users.
We will keep this post updated with the other features of the iOS Google Assistant. Note: As of now we count this news as a rumor as there is no announcement, we have from the Google regarding the same.