I'm not necessarily requesting that 0x40 be fleshed out and clarified: I'd be just as content to see it dropped. If, as I assume, the 0x40 is just the same as the 0x50 with a different (human) interpretation, then perhaps we should just drop it. If people want to assign human interpretations to their signatures, let them use notations.
As I remember, it stays there for the same reason that some other seldom-to-never-used
signature types are there: for backwards compatibility with their never being used. They are there for the same reason there is old stuff in my garage -- we hope to use it someday.
I'm not sure spring cleaning is warranted, but it's easy enough, if people think so.
If it can be marked with a SHOULD NOT use / deprecated then that would be good. More spring cleaning is better. OpenPGP's incessant algorithmic messiness slows its migration.
(a general comment, not specific. for clarification I have no clue as to the particular number.)