What is Deism?
For those that don't know, the definition for the term is
Belief in the existence of a supreme being, specifically of a creator who does not intervene in the universe. The term is used chiefly of an intellectual movement of the 17th and 18th centuries that accepted the existence of a creator on the basis of reason but rejected belief in a supernatural deity who interacts with humankind.It is worth keeping this definition clearly in mind because there is a lot of obfuscation about Deism out there, much of which I now suspect is intentional.
Is Deism a form of theism?
Only in the very loosest sense of the term, but this is really an interesting point to bring up. If what I've read on the matter is accurate, it seems that the terms "deism" and "theism" were essentially synonymous, with the term "deism" preferred, until Deism became a thing in the 17th and 18th centuries. The terms diverged at that point, leaving theism to refer to belief in the existence of a supreme being that does intervene in the universe.
That many religious people, particularly apologists of various kinds, are eager to include Deists in their rolls tells me that they're getting fairly desperate for the appearance of wider belief than is warranted. The reason is clear: Deism directly contradicts every brand of theism on the does not intervene in the universe point.
It is, in my opinion, therefore best not to consider Deism a form of theism. Particularly, whatever religious apologists might try to have us believe, the majority of people who believe in God (generally: theists) do not believe in the Deistic God. They believe in a living, breathing, interacting God of this world that answers prayers, performs miracles, and will one day judge the living and the dead. If you have lost sight of this fact already while reading this, let that be a signpost for you to realize how quickly and easily apologists can get into this Deism or philosophical stuff and make their audience forget entirely that what they're talking about isn't what they're really defending.
Can Deism be refuted?
I think it is likely that the answer here is "no." I'm quite sure I cannot do it at this point, although I've spent some time trying. The reason I suspect that the answer is a fairly unequivocal "no" is because Deism is potentially unfalsifiable and, at the very least, likely to lie directly on the other side of any coherent epistemology that we can create. Do note that the flip side of this statement is that if Deism cannot be refuted, then it cannot be proven either. Deists, and religious apologists depending upon arguments for Deism, would be wise to take notice of this fact.
Should we accept Deism, then?
No, I don't think so. I certainly do not, even though I cannot refute it. I'm perfectly comfortable dismissing Deism because it's (very nearly) good for nothing, and that which it is good for, so far as I can tell, can either be done better in other ways or is outright harmful.
On what grounds can we dismiss Deism?
It's useless for any honest use.
What is it good for that could be done better?
Deism stuffs a proxy into a hole in our knowledge about how the universe came to be, if that wording even makes sense to use. That proxy doesn't answer any questions, although it gives the appearance of answering this fundamental question about the nature of the universe. A good analogy for Deism in this case is that we have a box with a round hole in it, and wanting for a round peg, we stuff the hole full of cotton balls so we can call it filled.
Because of the general utility to mature, informed thinking presented by being willing to accept having no answer to particular questions, dismissing Deism may, in fact, be a net benefit to people, especially those who are Deists primarily to stuff cotton in the "where did the universe come from?" hole. (And is there any other reason to be a Deist?)
What is it "good" for that is harmful?
Making philosophical-style arguments. The cosmological arguments, e.g. the infamous Kalam Cosmological Argument, are arguments for Deism, not any brand of interventionist theism, whatever an apologist might want to try to sweep under that rug.
This is harmful because, since it is unlikely that Deism can be refuted, it gives a veneer of plausibility to the existence of a "supreme being" that "created" the universe and everything in it, that being always being called "God." This convention people use is very convenient for religious apologists who want to state that "'God' exists" and then use that statement to slip into "my God exists."
I think it would be an interesting thing to see if the Deist God was given a completely different name than "God," maybe "Dod" or something like that. That wouldn't erase all of the apologists' slip, but it would make it harder to apply.
Because the creator "God" idea is tied to the God-concepts behind most brands of theistic religion, misusing the term "God" to apply to this Deistic idea gives false credence to the unsupported claims of theism, and so the harms that come with theism and theistic religions are partially predicated on this bit of verbal imprecision and intentional equivocation.
Deism is not exactly the same thing as theism, and while a supreme creator of the universe may be a necessary feature of theistic religions, it is not sufficient to justify any of them. It is unlikely to be provable or disprovable, but ultimately, the Deistic hypothesis is (slightly worse than) useless and should therefore be dismissed after the requisite consideration. Beware any religious apologist trying to pull and argument for Deism on you as if to convince you of the beliefs and claims of their particular religions. At best, the tactic is extremely weak, and at worst, it's outright dishonesty.