Not getting regular erections will in fact cause your penis to shrink. There are medications you can take that will make it is hard to get erect. However. It’s not going to shrink an insane amount. Maybe half an inch, MAYBE a lil more. However, there is often a problem with this. 1. Any medication, especially taken long term will likely have other unwanted health consequences. 2. If it is hormonal, it will likely decrease sex drive to the point of not even wanting sex, so it won’t even be exciting being smaller. 3. Unless you stay on the medication (which likely will be dangerous), the shrinkage will not be permanent.
These are facts, you can look them up that not getting regular erections can in fact shrink your penis.
My penis in its flaccid state normally is 1.5-2” long and erect around 3.75” long when super turned on and 4.5” around. I took a medication for a few months for some health related issues. It made it so I didn’t really get erections. I no longer had morning wood, I didn’t get random erections during the day or even semi erections. I could still get hard if it was played with, but it took much longer and often wouldn’t stay hard for long. My sex drive was very low as well. My soft penis didn’t get shorter but always stayed at its shortest around 1.5” and looked a little thinner. My erect penis shrunk to about 3.25” long and 4-4.25” around. So it wasn’t a major shrinkage, but it was a shrinkage. After I got off the medication, it took about 1 month before my sex drive and everything had returned to normal. Erections over those few weeks started becoming normal, and I even started waking up with morning wood. Within that month my penis was back to its normal self. Even my wife noticed and said I felt thicker in her hand (she didn’t mention anything about the length).
So I’d assume other things that made it so you didn’t get erections would temporarily shrink your penis, but any changes are going to be temporary