Hi Alex, no hard feelings. Every body has its own intentions spending too much time over here. 'Odd' access tags and values appear in my searches hoping to improve automated routing by whatever third party software that uses osm-data. That's why i came along.
For me, optional_sidepath equals to: "if you like you can use a sidepath". That sounds the same as a legally foot=yes for the Leinestrasse. In Germany that is considered to be the default, so basically it is not really nescessary to add it any way. However i personally  do prefer to be as explicit as possible, so foot=yes would do as well. Splitting the road and tag foot=use_sidepath is the most detailed way of tagging and probably fits best in your precise approach. I leave it up to you.
Concerning the bicycle=optional_sidepath, it is often used wrong in my opinion. Because often there is no sidepath available or it’s arguable considered just to be ‘dangerous’. Access/motor_vehicle/bicycle/foot should describe the legal access, not the preferred. For these cases i would say it's a legal: bicycle=yes + class:bicycle=-1/1/2/3. But you are right that there are still a couple of thousend of them and described in the wiki, so to be considered 'in use’ :-). Kind regards, Joris
