Today I was revisiting an old project that used opto-isolators for UART communication and couldn't work out how it had ever worked until I realised that Rx input on the Nano has a 1k resistor to the FTDI chip Tx line that effectively behaves as a strong pullup.
As a general rule do modern boards still have this resistor, or is it a case of some do, some don't?
I would not call that a pullup, though it does act as one when the serial port isn't sending anything., it's more of a protection resistor - so that when you aren't using serial and you use the pin that's normally Rx as a general digital output, the pin isn't fighting with the serial adapter's Tx line for control of the line, driving it in potentially opposite directions, leaving the voltage somewhere in the middle until the pin drivers on one side or another burn out.
Putting a 1k resistor in there limits the current that flows in that situation to a fully-within-spec 5mA.
This resistor is ubiquitous in serial adapters, (I think I have bought maybe only one design of dirt cheap chinese serial adapter out of over a dozen - that didn't have a resistor between the chip's TX and the exposed pin - people screw up with wires and accidentally connect Tx to Ground and stuff like that.) That pin is (or damned well better be) present on all boards that have an USB-serial adapter built in....
Compare the schematics of a genuine Arduino Nano (ftdi) and a clone with say a CH340 USBTTL Chip.
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