As others have stated, Ubuntu was never a minimalist distribution.

When Ubuntu first came out, most common CD drives could only work with CDs of a about 700Mb capacity, but assuming that the tech would keep improving, it was reasonable to suggest this. It was never because Ubuntu was touted as “lightweight”, though. Then writable DVDs became almost ubiquitous, and they eventually migrated to that. In the meantime, USB flash drives were getting cheaper, and Linux installer tech was also improving, so eventually (quite early actually) these were supported too. But this was simply a logical progression of tech development and catering to the predominant baseline capacities of storage media.

If you want a proper minimalist, try PuppyLinux or DamnSmallLinux. If you want to run on a Raspberry Pi (or one of their other supported SBCs) try DietPi.