My Gamin Connect Mobile (Android) updated this morning and I am happy to report that this is working for me now. "Text Responses" now appears in the list under devices / fenix 5+...right below Garmin Pay. I've played with it a bit this morning and it seems to work like a champ. I'm running the beta 3.53 on the watch and my Garmin Connect Mobile is showing as version 4.9.2 updated July 26th.
In case this helps... I thought the same thing at first regarding incoming call responses, then I noticed that it is limited to just three responses. Since there are already three by default, I had to removed one to be able to add others. Once I removed it, the option to add other incoming call responses instantly appeared.