I guess this is an interesting contrast to Windows, where not only certain characters (like ? or * or |) are banned, but also entire filenames that used to refer to device files in DOS (con, prn, lpt1, etc.)

Not really. . and .. are the only standard directory entries that are added by the system.

Some shells may extrapolate from that by adding ... to go two directories up, but ... can just as well be the name of an actual file or directory.

Think you misunderstand me. Long before texting was a thing, landline phones (with rotary dials!) also had letters associated with digits. This layout was later transferred to keypads, which in turn became the SMS layout.

I bet this is also why old-timey phone numbers encoded the first digits with letters. The US had the famous ABC2/DEF3/… system that’s still displayed on most keypads, and the former Soviet Union mapped the first letters of the Russian alphabet (skipping З to avoid confusion with 3)…