Oops! I did forget to specify which calendar application I am using. Thanks @zkarj for pointing that out. I am using Apple’s Calendar App (with iCloud syncing).
I’ve checked all the settings within Apple Calendar that I can think of. I can find no option to manually control retention history as @simonsmark describes. This, to me, suggests that JaneApp calendar subscriptions may not allow for retention. Thank you for sharing your experience @simonsmark. I have no other calendar subscriptions, so I had no way of comparing subscription experiences.
I have been keeping a local copy as @quorm suggested. Unfortunately, the duplicate option is not available for my calendar subscriptions from JaneApp (the option is greyed out/no dice with cmd-D). So I end out have to manually create a new calendar entry for every appointment. This is a tedious waste of my time, but may be my only option.
I do know that the practitioners are not deleting appointments from their records. I can also see the appointments listed in my history within my JaneApp accounts. I have reviewed the JaneApp support but did not find any information specific to calendar subscription retention. I guess I will either have to live with this or reach out to JaneApp for help.
Thanks everyone for your help!