Once IOS 18 is released, will I be able to send a text message to an Android phone using WiFi instead of having to have cellular service?
IOS 18 iMessage to Android
I don't think this has changed in a while, and AFAIK won't change in iOS 18, but SMS is sent over a carrier's network not Wi-Fi, but it's up to the carrier to decide how to route an SMS.
iMessage messages are sent via any data connection, so cellular or Wi-Fi.
Some carriers support SMS over Wi-Fi, but that's up to your carrier.
If you're thinking about RCS, well that might work, but again your carrier has to support it for it to work.