What happened to the reg_uart_clkdiv ? I am trying to develop a UART test, and the reg_uart_clkdiv seems to be commented out as if it isn't available at all.

The developers of the VexRISC version of the processor (that is, the openlane developers) switched to the LiteX UART, which has a fixed baud rate that cannot be changed (they also switched to the LiteX flash controller, which is also limited and cannot switch between standard, dual, and quad modes). Claire Wolf had an excellent simple UART design with lots of control options including a wide range divider for setting the baud rate (and also the excellent SPI flash controller design). The choice of the VexRISC took a perfectly good processor and peripherals optimally designed for small microcontroller use, and replaced it with crippleware (over my protests, because I've always preferred the picoRV32). The more straightforward and less aggrieved answer to your question is that

reg_uart_clkdiv

is left over from the picoRV32 implementation and nobody removed it. It currently points to what I believe is an unused area of the memory map.