I would cut the line, about 8-12” from the T, Bend/fold the plastic supply, 2-3 times and crimp, I have copper wire, I would use existing copper wire to crimp the plastic supply line.

https://www.hunker.com/12490072/how-to-cap-a-copper-refrigerator-water-line

I would cap it with the proper size brass cap, which should be available at any hardware store; take that photo with you so you get the correct part. Does that piping have a shutoff valve nearby that is not in the photo?

Yes, the shut off valve is actually facing the wall for some weird reason. You can see the sides of the valve if you look closely at the picture.

Aha! I see it.

Well you could simply shut it off and disconnect the plastic line. But it would be safer over the long term to also screw on a cap in place of the fitting with the plastic line in case the valve seal started to leak at some point.

Assuming that the tubing is polyethylene, a John Guest type tubing cap can be put on the end of the tubing. This type of fitting has its own o-ring for a seal.