For what it’s worth, I like “open onion services” for the reasons Alec stated. “Known” and “overt” seem fine to me, but I like the sound and connotation of “open” better.
Personally, I feel like 'open' has to broad of a set of overloaded meanings and could be confused for one of the other meanings that is not intended. If these are 'open onion services', does that make the others 'closed onion services'?
How about “exposed" onion services? It’s less ambiguous than “open”, is an antonym to “hidden”, is short and nice to say, and suitably indicates a lack of protection for the onion service.
My vote is for the most obvious, because its name positions it in relation to existing onion services: 'non-hidden onion services'. The traditional onion services would then be known as 'hidden onion services’.
First, I hope that long-term we could move away from the term “hidden” because it has a negative connotation (I like “protected” as a positive replacement). Second, “non-hidden” is annoying to say and hear. Third, I’d prefer a positive term to a negative, that is, one that describes what these services are rather than what they are not. Exposed onion services should be considered to be equally valuable to hidden onion services and not their lesser, deficient cousin.
Aaron