Is it possible to install Ubuntu touch (UBPort) on the L5?
Perhaps turn the question around the other way … does the UBports project list the Librem 5 as supported hardware? https://ubports.com/en/supported-products
Another question would be, regardless of whether it is supported or whether it works: has anyone tried it?
If you yourself intend to go down this road, I would try putting it on a spare uSD card and booting from uSD card (requires up-to-date uboot). That way, if UBports is epic fail on the Librem 5, you didn’t mess anything up.
There was an effort to support Ubuntu Touch, as announced by Purism. Some work was done, as evidenced by commits and images - don’t bother trying these, they did not really work on my Librem 5 Evergreen.
Generally, as seen with the PinePhone port, running Ubuntu Touch on a mainline device, introduces some issues not seen on Android devices with hybris/halium. If you want to port Ubuntu Touch to the LIbrem 5, the recent community port to PinePhone (Pro) may help.