A new proposal makes its way into the RFC: It aims to deprecate the cycleway=opposite tagging family that is associated with (logical) problems and is considered outdated by most mappers, since meanwhile there is the much more widespread oneway:bicycle tagging allowing to map the same meaning in a better way.
But that was specific to its use with highway=cycleway. The previous description implied that is is also used on other types of highway=, and did not say it was deprecated or obsolete. The proposal that @Friendly_Ghost linked to does deal with these two different situations, but seems to have been preempted by this change to the main cycleway page.
I see what you mean. I looked into it, and based on Taginfo, 43% of cycleway=shared is used on highway=cycleway. I can see why that would be obsolete. That still leaves the usage of this tag with other highway=* tags unexplained and undocumented.