can handwritten messages only go to iphones or to all mobile phones?

You're correct sodpark.

IOS 11 ate the feature that did in fact allow handwritten messages to be sent to Android users is gone. During the time i ran ios 10 i sent them all the time to Android friends and family. Updated to 11.0.3 and it's gone. The most fun thing is taken away. Apple won't tell you this. In fact Apple will say you were never supposed to be able to send to Andriod, which is false. Finally the Senior Consultant and an Apple Genius who did not believe me reproduced it themselves.

I had this same question after I updated to IOS 11. With IOS 10 I was able to send handwritten texts to my wife's Android . Not so now .

It uses iMessage, so only to Apple products. See the following from Use message effects with iMessage on your iPhone, iPad, and iPod touch - Apple Support

You need iMessage to send message effects. If you aren't sure whether you're using iMessage or SMS/MMS texts, learn the difference