While it is possible to write a software UART, such a solution
imposes fairly severe limits on the rest of the software. The
software UART requires cycle-exact timing to work, which means
that no interrupts are allowed while it is receiving.

There are other ways of tackling the problem of too few UARTs,
for example using a chip that comes with more than one UART, or
connecting external UARTs (for example via SPI, but there are
other options).