Given that there is no need now for one to swith off their cellphone midflight, inflight wifi is now a norm and common, Is it possible for me to measure the velocity and altitude of an aircraft using my personal car GPS speed indicator or cellphone app in the case of speed and altitude as a passenger?
Depending on the reliability of the sensors in your phone, yes that sounds completely possible. Most modern aircraft use GNSS for navigation, and your phone relies on the exact same technology.
Be aware that you will get the ground speed - see this question for further details: Why is there a difference between GPS Speed and Indicator speed?
Yes. Airplane mode generally only includes transmitters, and GPS is receive-only. The below is a screenshot I made with a speedometer app, on an Airbus A320. I had to hold the phone near the window to get GPS coverage. (Units: km/h groundspeed)
On a more recent flight, I could even get GPS coverage away from the window, in a Boeing 737-200 (although I was seated next to the overwing exit).
Note that some altimeter apps may use the phone's barometer, which will only give you the cabin altitude.