> When did method chaining become 'functional programming'?

As soon as you stop calling it "method chaining" and start calling it "function composition".

If you chain together a bunch of methods ('.' operator) in an OO setting, that's called a "fluent interface". It's a sign of good design and is to be commended.

If you compose a bunch of functions ('.' operator) in an FP setting, it's an unreadable mess, and you will receive requests to break it into separate assignment statements and create named variables for all the intermediate states.