Hi!

No, generic footways should not be marked with `bicycle=yes` if they don't have some kind of signage. All footways in Latvia follow the same legal traffic laws. By default, bicycles are not forbidden on footways and there aren't any special laws about it. So `bicycle=yes` is the default value on all footways. This is similar how we wouldn't tag `foot=yes` on every footway or `car=yes` on every road. Unless traffic is forbidden or the way is specially designated, we simply don't add any extra tags. So it is up to the map renderer or routing engine to decide how to treat this based on what the user wants. Tagging something with `bicycle=yes` kind of implies that there is some sort of dedicated or special consideration for bicycles or at least we want to be very clear that bicycles can use something that would otherwise be ambiguous (I won't go into exceptions here). 

More info about cycling tagging: https://wiki.openstreetmap.org/wiki/Lv:Latvian_tagging_guidelines#Veloce%C4%BCi_un_veloinfrastrukt%C5%ABra