Dell still sells the XPS 13 with Ubuntu just not where you possibly reside (PL?). I was actually looking at their DE website the other day where it's available, but they only ship to Germany.

Here's a thread I found about the XPS 15: https://askubuntu.com/questions/1042414/trying-to-install-ubuntu-on-dell-xps-15-9570 . It seems the experience is not smooth.

I set up Windows 10 Enterprise onto a Dell XPS 9560 recently, and even doing that was beset with difficulties. Dell seem to have reserved part of the SSD drive for their recovery software, and it is write-protected from re-partitioning/deleting that bit of drive. Then you need to turn off Secure Boot / remove the Secure Boot keys in order to set up custom, corporate Windows (similar to what you'd need to do for Linux). They're nice-looking machines, but I'd tend to seek out something more practical for myself and Linux. [For instance, I like built-in ethernet port and I like to have easy access to upgrade/replace SSD, RAM, Battery.]