Sandy roads which bicycles can´t use

Thank you - and that´s the problem. I think there´s no option to describe a road as "possible/transitable for pedestrians and cars" but "not possible/transitable for bicycles" - so maybe this would be a suggestion for a new option/button/caracteristic of roads

Well, one reason there is no such tag as "not passeble" by bikes is that that is a pretty subjective matter. Roads you as an experienced biker still would use others would already avoid. On the other hand some bikers with fat tire MTBs would still want to go via the ways you describe in your question. That's why we try to describe the situation by tags like surface, smoothness, trail_visibility, tracktype etc as accurate as possible and leave the judgement to the user. I know that is difficult.

I remember riding along a beach and finding the damp sand was firm and we could make headway but the dry stuff was almost impossible to cycle in/over. So maybe surface=sand could be useful for users to decide to use or not.

agree - surface=sand would be the most important thing to tag. E.g. the default cycling profile of the routing engine graphhopper assumes a maximum cycling speed of just 6 km/h on sand (compared to 16 km/h on compacted) (if I read correctly). This will heavily impact the routing over such sandy ways.

An important parameter for sand surface is how thick the loose sand is. If it is just 0.5 cm sand on the surface and below something hard, then it is quite fine to cycle on. If it is 10 cm loose sand, then it is quite impossible to cycle on (unless you have really fat tires). surface:thickness (attention - current use: only in Afghanistan for some big roads with mostly surface=asphalt)?

smoothness is also fine to add, but in case of a sand surface its value assessment is quite hard due to the described surface.