If you receive many offers, I suppose it would be a great job, unless there is some automatism or you want only send this messages to certain freelancers.
I appreciate when a client sends me a rejection message and, in particular, when he/she says that he liked my portfolio and may take me into account later.
It seems to me a kindness on the part of the client.
Yes. Most of my jobs are invitation only, ten or twelve maximum invites. I acknowledge each applicant and keep everybody abreast of the selection process. When I get to the end and can only hire one, I let those who didn't win know, and try to tell them what they did right and what I thought could be improved in their application. I've never had the pleasure of inviting the freelancer who feels offended, and never will.
If a client is overwhelmed with responses, it's the client's fault for not doing a better job of screening applicants. It only takes a couple of minutes to say, "Andy, thanks for your response. I chose someone with more experience for this job, but I start every job from scratch. I found your pricing to be somewhat low; if you don't think you're worth very much, you can't expect a client to argue. I recommend you raise your rates. The winner asked me pertinent questions in her response, demonstrating a real interest in my work. A few questions to demonstrate your interest and understanding, and I might well have chosen you."
Andy D wrote:
I would like them to want to contact me anyway later if I propose other work.
Should I tell them that I refuse their offer?
Especially if you never interviewed them, you don't need to contact them just to tell them that you rejected their proposal and why.
Upwork already understood your concern and hence offers you a reasonable level of anonymity as a client, until you interview, and then hire someone.
If you didn't interview, or never went ahead ahead to hire them, they might never know that it's the same you who posted that job subsequently, and will very likely apply if they like the gig.
You will do what you wish and get results appropriate to your actions. I want to work in a community of professionals, so I treat clients and freelancers with respect due them as professionals. If I invited the freelancer, she/he knows who I am. I invited the freelancer because I believed s/he was qualified and understood business. I want the freelancer to feel good about the interaction with me.
Regarding the role of client as "doing whatever the rules allow you to do" won't help either the client or the platform. That's a race to the bottom.