Hm well, there are several issues with this approach…
First, the shoulder tag only declares the physical layout of a shoulder. It does not communicate anything about how it is used or usable (in reality), other than that you can stop there if your car broke down.
As @Minh_Nguyen noted, there could be all kinds of stuff on the shoulder that inhibits it being used for cyclists. E.g. it could be used for parking parking:lane:parallel=shoulder (/parking:position=shoulder)
Additionally, pretending the first point is not an issue, there is a problem with the level of detail (for data consumers). Just to know whether there is a shoulder usable as a cycle lane, a data consumer so inclined would need to analyze three tags. More importantly yet, it is even more so of a problem for the mapper (of shoulders), because to make the shoulder information usable in the context of cycle infrastructure mapping, he’d have to add up to 3 tags. And that, on the premise, that mapper knows about the relevance of such precise shoulder tagging for cyclists - i.e. why should the mapper care about adding such detailed information?
And lastly, well, we have to work with what we got: Over 20000 ways have cycleway=shoulder tagged. Removing these would mean to destroy previously collected data. If cycleway=shoulder is something that should be removed, then there must be an equivalent tagging with which the information carried in this tag - “cyclists can use the shoulder” - can be expressed instead.
Even with a reformed shoulder tagging (that could just™ be proposed) as discussed in the shoulder-thread, there’d not be a 1:1 tagging conversion to it, as the minimum information that must be supplied there whether it is a wide or narrow shoulder (which is irrelevant for shoulders usable by cyclists).
It’s really quite similar to parking:position=shoulder, the more I think about it. Even from a reformed shoulder tagging, you can’t really infer whether cars (can) park on it. After all, it could be a narrow shoulder, but the rest of the verge is filled with gravel, so shoulder parking is possible after all (or would that be half-on-kerb-parking?) Or, it could simply be prohibited.