11111111 is technically possible to use, but it would cause some problems. Sending it over the wire would break telnet, for example. Also since we already introduced 11111110 for 7-byte encodes, we're getting dangerously close to making the UTF-16 BOM character (11111111 11111110) accidentally show up in UTF-8 (this is also why 11111110 wasn't in the original maximum-6-byte UTF-8 spec). I still don't think it's possible to have the UTF-16 BOM show up in our hypothetical extended UTF-8, since 11111111 could never be immediately followed by 11111110 (or vice versa) in a well-formed UTF-8 stream.

Also note that if you did add 11111111 as a valid head octet representing an 8 octet long encoding, you'd still only have 42 usable bits (since the first byte is still entirely consumed by the length indicator)