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Ideally you wouldn't need to expose a TTL serial debug port to begin with. Maybe on a prototype you would want this but I'd rather just have a single connector that can expose everything (jtag or swd). Bonus points if the interface chip is on the board so it's just a USB port

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FUSB307B is amazing for this, it has a hardware DBG output pin indicating that a USB-C debug accessory mode connection has been made. Perfect for connecting a mux and putting SWD & serial on some USB-C pins.

Too bad there's no standard for debug accessory connections. Also, at that point (putting a USB-C TCPC on your board) you might as well do full usbserial…

NB: there's no orientation detection in debug accessory mode.

Well the first problem you will hit I'd that very likely you will need to protect or isolate those lines from ESD. This will raise price of device and it will get denied just in these grounds.

ESD diode costs are nothing compared to the connector headers or even dealing with the mechanical complications of exposing Just One More Thing. Seriously, they're cheap.

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