Disable (dead) phone number? Retaining for historical benefit. Is this possible, how can I request this feature? I am hoping to find a way to keep but disable phone numbers in Google Contacts.  

For example, several people I know have at least 6 numbers in my contact for them, where often of those only a single one is the "right" number, where many of the others are their former mobile numbers.  

I prefer to retain the (now dead / invalid) mobile number(s) on the contact so that this user's texts / calls / activities that I backup via apps all "stay together", but at the same time, I would like it to be impossible to call or send a NEW text to that number (since it is marked as "former" or "disabled" or "retired" or something to that effect).  

This has been something I've been searching for for quite some time, but it has recently become more important as my sister forwarded a picture of my niece I'd taken opening Christmas presents from my phone to hers, not seeing that she picked the wrong phone number (for herself), and in so doing sent the photo to a totally random person (who simply by chance has been assigned her old mobile number).  This is the scenario I'd very much like to prevent.

Of note, I'm using a Nexus 6 w/ latest (Marshmallow) OTA Android (6.0.1).  I use mostly stock apps with the exception of VZW Messages for SMS.

I believe that this would need to be an OS-level feature for it to be effective, and I have to imagine other people have wanted the same thing.
Its easy to keep multiple inactive numbers; just place them in NOTES for each contact.  Notes is a wonderful item available on each contact.  I keep tons of data there and never run out of space