path overgrown by trees - Now i want to mark that part

It is quite a common problem and our walking group tend to take pruning shears and sticks so we can snip and bash brambles before path become impassable. I would add a note like this one. https://www.openstreetmap.org/note/1966102 ( following comments this note has been resolved) I have added a note to the map data now. The note describes the under and over growth. Incidentally The Cambridgeshire County Council should clear dedicated paths in this area (rights or way in Cambs England) periodically but they have been starved of finance by national government and it's not considered a priority. I would NOT want to mark vegetation as a barrier, it's usually possible to get through, A barrier in the map data could reduce use even more and the path could get even more overgrown and impassable. I realise that you are talking of cycles but i have pushed or dragged my bike through some rough stuff in the past. Have you seen cyclo-cross.

Would you have been able to progress at a reasonable pace by foot if you didn't have to drag a bike behind you? If so, re-tagging the relevant section as highway=footway might be appropriate.

If it is effectively blocked, some sort of barrier=* on node placed on the way in roughly the correct location might be a useful way to tag that sectoin as unusable to routers, renderers etc. It may also be a good idea to leave a note=* or a fixme tag explaining why you have added this to future editors.