A similar question (although specifically mentioning lichen sclerosis) came up in the facebook group just recently, which I responded thus...
There do seem to be some chains of events which prostate cancer patients should be aware of...
Treatments often result in loss of erections.
Some treatments and/or loss of erections leads on to penile shortening.
Loss of erections does sometimes seem to lead on to phimosis.
Penile shortening and/or pubic fat growth can cause "buried penis", making standing to pee impractical.
Phimosis or buried penis leads on to inflammation because cleaning becomes impossible.
Inflammation and/or constant urine contact due to incontinence may cause lichen sclerosis (I don't think this is fully understood).
Lichen sclerosis is a known cause of penile cancer (although I haven't come across a case starting from prostate cancer treatments).
I think it's important men keep their penises working properly, even if they have no interest in erections or sex. They probably still want to be able to stand to pee, and to keep themselves clean and inflammation/infection free. That probably means any man who isn't getting regular spontaneous erections should be regularly using a pump for penile physio, again regardless if you are interested in erections or sex.
If you do have phimosis, simply stretching things can make it worse by causing micro-scarring and more fibrosis, so you will need to get that sorted by going to see your GP or urologist before you start using a pump. Steroid creams sometimes help, but if that fix doesn't persist when you stop them, circumcision might be required. (I don't think you can simply stay on steroids forever without getting other problems.)