There's an upcoming API from @meganrogge TerminalCompletionProvider that allows extensions to provide completions for the terminal. That's how the built-in extension does this:

Combined with the shell type API, it should be possible to support completions in Python REPLs! We could do this basically by putting an adapter on the existing LSP with a "fake" text document made up of the terminal command line and cursor position (maybe more context later?).

@anthonykim1 and I put together a quick proof of concept that got completions showing a month or so ago.

API:

https://github.com/microsoft/vscode/blob/1fec88a0a63df190caf637357740919de96f75cb/src/vscode-dts/vscode.proposed.terminalCompletionProvider.d.ts#L1-L138