What are the rules / best practices around adding disclaimer text to survey or email text?

I did some online searching for best practices around adding disclaimers to surveys, but I was underwhelmed by the available content on this topic. Are any of you experts in this area or can you point me to a resource that covers best practices for how business should use disclaimers in surveys? I'm familiar with rules used at universities around the protection of PII, and I'm aware of a lot of the GDPR rules. I'm just not savvy on how to set up disclaimer language or if it's required that I set up disclaimer language or consents for every survey that goes out to customers.

I know that most of the surveys I take from other businesses/websites/service don't precede the survey with a disclaimer. It is my assumption that if I have a login to a site and they give me a survey, then they will be tying my response back to my PII and use it internally.

What I don't know is this: If you send surveys to customers who have logins to your product already (and who have already been covered by the NDAs and purchase agreements connected with using your product that tell them that we will use their data to improve the product), then do I still need to let respondents know that their responses are not anonymous or that their PII won't be shared with people outside of those in the company tasked with improving customer experience? Are we legally required, by GDPR or some other rules out there, to let users know every time we are going to be tying their responses back to their PII internally?

Any thoughts on this would be welcome.