Someone think they could implement https://imihajlov.tk/blog/posts/eth-to-spi/ on GF180MCU or SKY130?

You need a differential input for 10Base-T. Currently for GF180MCU padring is fixed, so you'll need an external differential to single ended convertor, something like this https://www.fpga4fun.com/10BASE-T4.html or an external transformer. All other parts of 10Base-T transceiver and MAC could be easily implemented. I think that several Ethernet channels could fit inside user project, so not only SPI-Ethernet or Caravel MCU with ethernet, but also a small 10Base-T switch is entirely doable. On SKY130 differential input probably could be implemented onchip using Caravan environment.