Flexible! The first byte tells you how many bytes of addressing you have. Perfect and future proof!

At best future-resistant.

True future-proofing would require representing address length as an arbitrary-precision nonzero unsigned integer.

Since allowing a zero-length network address format would serve no purpose other than to pointlessly complicate standards definitions, you could trivially and without loss of generality interpret zero to denote some extended-length address length representation to be defined in a future version of the standard.

Hardware implementations typically do not like variable-size fields. Not just because the total header size becomes unpredictable, but because it means any following fields no longer have a fixed offset, and that complicates parsing.