MacBook Pro with headphone port doubling up as audio line in port

No recent MBP models have analog audio line in. In the past, certain models supported earpods with a mic and used a specially-wired TRRS jack. Even then, the input was mic-level, not line-level.

You will need an analog-to-digital interface. There are many on the market. What analog audio source(s) do you want to use with your MBP?

AFAIK, none of the current MacBook models does line level audio input directly.. It's been a number of years since that was an option.

Your best option today is to use some flavor of USB audio input dongle. There are so many to choose from at so many price points.

Give us an idea of your end goal and someone will point you in the right direction.

There are many external audio interfaces on the market – ranging from inexpensive USB "sound cards" (probably what you're looking for), to a Thunderbolt console with 32 audio inputs and 32 audio outputs that goes for well in excess of $10,000 – and is obviously intended for use in commercial recording studios.

There is no getting away from using USB interface. The fact is it's an efficient way to add functionality and capability to any computer. Yes, more devices mean more potential points of failure, but there's just no way to build into a computer all of the functionality that all users might like to have.

The best we can do is buy quality devices and cables and demand from all manufacturers that they build their wares to high standards.

It's always best to use appropriate cables from point-to-point rather than add adapters to the chain.

I don't disagree that older Macs had some nice hardware features like the analog+digital optical headphone outputs and separate audio lines-in for example. But I do enjoy the advances in processing power and portability of the newest Macs. I'd say the trade offs are worth it.