The issue with the word ‘coding’, in reference to a character being ‘black-coded’ or ‘autistic-coded’ or ‘queer-coded’, is that it’s one of those concepts that people don’t actually look into and so develop a definition for it based off of vibes.
Coding is the use of the semiotics of concepts to influence an audience’s perceptions in situations where actually presenting said concept is impossible, such as due to societal expectations, legal reasons, or in-universe logic.
The Hay’s Codes made it impossible to legally represent the queer community in a healthy and happy way, so queer-coded characters were used in place of actual queer characters. This is also why so many confirmed bachelor characters and pansy characters were used in television from the 50′s to the 90′s. You could make a character as gay as you wanted, so long as it was never actually confirmed and used as a joke.
Characters like piccolo, garnet, sapphire, and ruby are inhuman aliens, and thus can’t actually be black, so they are coded as black. Characters like Twilight, Peridot, and Donatello are all likewise inhuman, and so can’t be autistic, so they are autistic-coded.
You’ll likewise note that plenty of autistic-coded characters were not initially confirmed as autistics. This is because they were merely meant to be seen as quirky or socially awkward, and that it was only the fandom surrounding these series commenting on how much they relate to [insert character] that saw them being recognized as such. While characters that were initially meant to be autistic and confirmed to be so from the start tend to be a mishmash of stereotypes, thus making their place as representation questionable at best.
This is why the use of terms like ‘minor-coded’ are not only incorrect, but also offensive and insulting. People can be short. People can be naive and childish. People can be interested in things that aren’t ‘adult’.
Saying that sexualizing a character or entangling them in romance because they’re ‘minor-coded’ is insulting to people who are actually like these characters in real life. Hijacking the concept of coding for the sake of being offended by pointless bullshit takes away its power as a tool. Looking at real life people and saying that they aren’t allowed to date, or that someone pursuing them romantically is tantamount to pedophilia is controlling and completely ignorant.
Not to mention the harm it brings to little people, by earmarking anyone who might be interested in them as a creep just because they’re a little person. Not dissimilar to the amount of people who see an asian person with a white person and immediately jump to fetishization...
Edit to Add: It should go without saying that you likewise can’t minor-code by dint of minors being fucking everywhere and in damn-near every genre. Like, clearly porn is an exception, and fucking hell is it fucked up that there are minor-coded roles in porn, but generally, a series can and will just... add a child if they want a child-coded character... Except for fucking Chappy, but I feel that Chappy’s more the exception that validates the rule.