Can a SIM card be used for data only on iPad?

Assuming that you have a WiFi+Cellular model iPad, the only type of SIM that you can use is a DataSIM.

An iPad, whether a Cellular model or not, is not capable of directly accessing Cellular Voice or SMS/MMS Messaging features. An iPad with either WiFi or Cellular connectivity alone can only access IP-data services.

If you have added a DataSIM and Cellular plan to a WiFi+Cellular model of iPad, the number associated with the SIM card is only used to identify your account for billing purposes. This number is not used for making calls or sending messages. An iPad cannot be used a standalone alternative to a mobile phone.

ericofcourse wrote:

Where does one acquire a DataSIM then? I have just learned that my data only sim card from an older smart phone will not work in my new-to-me iPad Air.

If using a physical SIM with iPad, you will require a DataSIM explicitly provisioned for iPad or mobile broadband (a.k.a. MiFi).

You can obtain a physical DataSIM or eSIM (if applicable to your iPad model), suitable for iPad, from your chosen Cellular Carrier.

Here is how to: Set up cellular data service on your iPad Wi-Fi + Cellular model - Apple Support

A common path is to follow the setup steps in Settings > Cellular > Add a New Plan

Where does one acquire a DataSIM then? I have just learned that my data only sim card from an older smart phone will not work in my new-to-me iPad Air.