Kinda similar to how AOSP has generic support for undo / redo in standard widgets like text fields, but most keyboards don't provide an undo / redo button and the ctrl-z / ctrl-shift-z keybinds aren't exposed via most virtual keyboards in their layout since they don't offer ctrl.
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All the Android keyboards are so bad. I use an abandoned one called multiling. That and hacker's are least-bad. A good one would provide blank keycaps matching compact pc physical layout and let you apply any xkb layout file on top of it.
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It's unfortunate that Google stopped developing their keyboard (Gboard now) as an enhancement to the open source AOSP keyboard project. They've made massive improvements which would have been included in it and it serves as the basis for most of the alternate keyboard projects.
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It's so ridiculous that "keyboard apps" (rather than keyboard layouts) are even a thing.
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It's support for alternate input methods in general. It covers the input method (typing on keys, swipe, more complex methods for other languages), suggestions, spelling correction, clipboard management (no longer allowed to be done by other apps) and approaches to multi-language.
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There are keyboards supporting configuring the layout. AOSP keyboard has internal support for it but doesn't expose much of it. AnySoftKeyboard exposes a lot of functionality for this, but it suffers from offering too much configuration without actually being a good keyboard.
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The basic AOSP keyboard is called LatinIME because it's the IME for Latin-based languages. Originally, Google Keyboard was directly based on it, which is still the relationship between a bunch of other Google apps and the open-source AOSP projects (like Contacts, Dialer, etc.).
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Originally, the only real difference between it and Google Keyboard was that the Google Keyboard version provided a library for mapping swipe patterns to possible inputs. AOSP keyboard supported swipe, but the library for doing the mapping was proprietary and not included.
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They eventually forked away from it and it eventually ended up becoming Gboard which has a ton of improvements. It has a lot of nice things like moving the cursor by swiping on the keyboard and way better suggestions, auto-correction, multi-language support at the same time, etc.
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It's really too bad they stopped building it on an open source base. In general, it means the engineers involved simply didn't care about making it open source. It's usually mostly up to the people working on the project. There are a lot of differences across AOSP due to that.
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Alternate keyboard support is really alternate IME support. It's not just for alternate virtual keyboards. I also don't think they expected there to be the interest in making alternate Latin-based virtual keyboards that there ended up being. They ended up offering many features.