didn't Epic Store give it away for free? maybe that has something to do with it not being here

No, that only happened recently and Celeste has been out since January 25th, 2018.

Here's another data point:

Many people on the Internet (including, for example, Youtube comments and on Reddit) say that Celeste helped them deal with depression/

In any cqase, it sounds like Celeste does not need a trigger warning the way games like The Cat Lady do.

Don't get me started on social media comments. We are past the point where you might find useful information in the comments. Most are just popularity posts milking thumbs ups. I gave up reading any comments. Even reddit communities are prone to this.

Sorry to bump, but has anyone been able to get an official reason why Celeste still isn't here? (other than, obviously, GOG's tiny marketshare)

Because everybody bought it as part of the recent bundle.

With today's disaster with Devotion, at least one developer no longer wanting anything to do with GOG because of it, and some articles mentioning GOG's past PR problems, I think I know what happened (though please keep in mind this is ultimately just speculation on my part and not an official answer):

October 1st 2018 we got videos that showed footage of Celeste. About three weeks later, the GOG Twitter account misused a hashtag intended to be about a serious issue for transgender people to promote their store in a way that had nothing to do with the hashtag's actual subject matter. This caused some backlash and bad PR, as we already know. But there's an important connection here: The protagonist of Celeste is transgender (don't worry, not really a spoiler, it's not mentioned but for a tiny visual cue at the very end of the game). Additionally, the main developer of the game would later come out as being non-binary in July 2019, which one could assume they probably knew a while before announcing it to the world. It's entirely possible that Celeste's GOG release getting cancelled was a direct result of that Twitter mess-up, making the developer no longer feel comfortable working with GOG or selling the game here.

Again, it's speculation, but it's the best explanation I can come up with.

O.o
The protagonist of Celeste is a child. Holy crap what repulsive creeps. Good riddance.

So, what's the problem with that?

(Also, I see no evidence of the protagonist actually being a child.)

Edit: Also, maybe you should play Secret Little Haven.

No she is not. While her exact age isn't stated, at the very start of the game she drives herself to the mountain and later there's mention of her drinking alcohol. She's clearly an adult.

... Or Glaswegian