I only have a fairly basic understanding of encodings, but is this one by any chance in one of the smaller “standard” character sets and not in the huge Unicode “emoji” set?
There's not really a "standard" character set and an "emoji" one. There is an emoji block (U+1F600 to U+1F64F), and this heart (U+2665) is not in that. But this heart is an emoji, in that the data file emoji-data.txt for contains `Emoji=Yes` ("=Yes for characters that are emoji", —UTS #51) for this character. However, this heart has `Emoji_Presentation=No`, which "=Yes for characters that have emoji presentation by default" (—UTS #51).
So basically, it should present, by default, in its non-emoji appearance. (Though there seem to be a number of readers for whom that isn't true. I think that's macOS being badly behaved.) There are some characters that can flip/flop between a "text" (normal monochrome) presentation and an emoji presentation; all of the card suit characters are. Unadorned (e.g., just U+2665 alone) then, you have to follow the emoji default presentation rules. In some contexts, such as web pages, that's "just do what that Emoji_Presentation flag says" for those characters, but see [§Presentation Style](https://www.unicode.org/reports/tr51/proposed.html#Presentat...) for exceptions to the rule. (HN is not an exception.) But you can also follow the character with a variation selector, to try to force either text (U+FE0E) or emoji (U+FE0F) presentation.
There are a number of what I'd call "yeah that's definitely an emoji" outside the Emoji block, too. Like U+2705 "white check in green box" (which HN disallows me posting); that one is Emoji=Yes but also Emoji_Presentation=Yes)
(IDK if Emoji=Yes Emoji_Presentation=Yes is HN's filter … but that'd be what I would do, if I wanted to filter out emoji like HN does.)
It may not be in the Emoji character block formally, but casually speaking it’s not wrong to call it an emoji, since emoji is basically just a picture character, and this is no less a picture character than what’s formally codified as ‘Emoji’ in the Unicode standard.
In addition to this, what I meant to convey is that it does show exactly like the iOS/iPadOS emojis, in color, with details, etc. Not the non-emoji font unicode character
They've always been stripped for me as well. This "emoji" actually appears black for me however.
I think the reason its not stripped is because it's actually "U+2665 : BLACK HEART SUIT {valentine}" which predates the emojis. Lots of fonts and OSes will render these older unicode symbols as emojis