(Well, it wasn't me, the comment I'm replying to now says 37 minutes ago. I did downvote your first comment just now, before seeing your self-reply. Sorry. I upvoted your second, to reimburse you.)

When I saw "I want to say" I stared at it for a minute..

I've been hearing it for a while from Yasser Seirawan (US grandmaster) doing chess commentary on major tournaments, on the St Louis chess youtube channel. He's a great storyteller and commentator, seems a wonderful guy, but a kind of walking verbal accident-waiting-to-happen. Like he seems incapable of pronouncing about 50% of names. He started saying "I want to say" frequently in the last couple of years. Lately one or two of his co-commentators starting saying it too. It seems to just mean "I think"—i.e. not "I think it was 1987" but "I wanna say it was 1987"—for no apparent reason except it's more words. (Maybe because no-one can say "You're wrong" to "I want to say"?) I really loathe it. Why tell me you want to say something, why not just say it?! Uh but it doesn't mean that apparently, it just means "I think". That was a perfectly good expression. "I want to say" makes me feel like going into a forest where I never have to hear humans speak again. (Related: I downvote any comment I see starting "Fun fact:"—that should be most strongly discouraged, I believe, being similarly barbarous.)

This is the first time I've seen it in print. I considered replying with a strong objection, imagined whether that could change the expression's course of popularity in English. Was considering favouriting or screenshotting it for further reference when I saw the comment I'm now replying to. Where does it "I want to say" (meaning "I think") come from?!

Apologies for long rant, but I felt I owed you an explanation! Maybe your other downvoter was also trying to nip "I want to say" in the bud on here, before it becomes another "literally" or "fun fact".

It's a venerable idiom that means something like "I think X, but am more uncertain than 'I think' normally implies."

This is now an aside, but I suspect it comes from this progression:

"I say that" -> "I want to say that"

It's a verbal form of softening what you're saying and indicating you're not sure. The "online text" version would be to use "IIRC".

Oh thank you, fascinating. Hmm I notice now that "I think" is similar to "I want to say" in literal meaning—not stating the uncertainty, but they're both like "Warning, about to report on what my brain is doing, don't blame me!"

It's similar to the third-personing we do when we have to enforce a rule: "I'm sorry but I have to ..." or "I'm afraid I'm going to ...".

In both cases we're trying to downplay our involvement a bit.