I never thought of null-termination as a "sentinel". I thought sentinel means strategically putting somewhere a value to avoid having to check for termination condition. Null-termination is not a sentinel, IMO. It is the opposite - terminator, which has to be explicitly checked for. Am I wrong here?

BTW. If you really want speed, SIMD will obliterate any non-SIMD solution, and my bet would be that most of the time SIMD will be easier to implement with length prefix because it will not have to handle a special case of null-termination. Someone correct me if I'm wrong.

Trailing zero is a a typical case of a short-signed hack. More naive, simpler, natural solution (length prefix) actually works better, especially long term, where other things change (hardware architecture, performance and resource constraint, use cases).