Do you have to stand at Parade Rest for a Senior NCO when not in uniform?

This falls in the category of "Common Courtesies". Its just like saulting an officer when you are in civillian clothes. If you plan on making the Army a career then you will some day understand. These are the little things that can take you to bigger things in your career.

Regulations pale in comparison to attention to detail. Be smart, be respectful, be sincere as it too is also a sign of respect. Respect is not black and white and no manual will encompass how to show respect. I assure you I can go to parade rest in a disrespectful manner if I so wished to. Will you catch me observing military customs and courtesies at a public airport? No way. Why do we not wear uniforms at airports anymore? We become targets, does not rendering standard military customs and courtesies during such a time render the no use of uniform null? Of course... we may as well tell everyone we are soldiers. There is a time and place for everything, and respect requires attention to detail.

So FM 7-21.13 The Soldier's Guide used to spell it out clearly in para 4-16. Now that it's TC 7-21.13 and not FM, it no longer says you have to stand at parade rest for an NCO or attention for an Officer. My question is, is it in writing anywhere else? If not, why did they remove that section?

edit: after further research I've found it in Tradoc Pam 600-4 23 June 2017, so at least I know they are still teaching it at BCT. I'm just surprised it's not in writing anywhere else being such a big tradition/ courtesy

It's a common sense question, if you run across any officer, commissioned or non, you render the appropriate courtesy. Do the same thing you would in uniform. If you pass someone you know at the mall render a greeting. "Hi sir/sergeant", and carry on. Chances are he's thinking the same thing you are, "why is he here?" or "Crap, please don't ask me about work." If you get nailed down assume the position until told otherwise and tough it out.
Maybe you're self-conscious because it's new to you but after a while it becomes habit. I've been out eleven years and last month, while hanging out at the local recruiters office the company commander visited, and when the office came to attention i was halfway there before I remembered I wasn't in the army.
TLDR: when in doubt, whip it out. You can't go wrong doing the right thing.

Yes. You are a soldier 24/7.

I don't stand at parade rest I just greet them and shake his or her hand and recognize them by their rank.

I'm retired 2 years now, first day on the job at my civilian job, the CEO walks into my office to say hello and invite me to lunch with him. I tell you I stood from my chair and snapped to parade rest like my Senior DI in boot camp had walked into the room. I caught myself at parade rest and had to put my hands in my pockets to keep from doing it again.

I even did it in Agfhanistan as that asshat (rank hidden to protect the stupid one) began to scream at me in front of the entire BDE Intel cell. Both case totally different, but in each case I showed respect and in return my peers in Afghanistan applauded me for restraint and at work my coworker acknowledge what I did. It's about respect, both up the chain and to your subordinates, it's too simple.

There is. Yes you do when is a dressing you or Any in ur group

Great question. Yes, especially if you know them and are they are in your chain of command. Being a PFC you should assume that you are out ranked in most cases. In due time you will get your respect. Best of luck to you in your military career.

I think it's always smart to error on the side of caution. If you know the person you are talking to is a SRNCO in or out of uniform, give them the respect of standing at parade rest. Even to this day, as a Veteran, I find myself standing at parade rest for people I know were/are SRNCO.