How should I tag river meanders (oxbows)?

I might add I’m not sure if there’s water inside, most likely is, but sometimes there are only bushes, trees, etc. Also some oxbows are in fact partially ditches, so I’m lost there. Also those old rivers are too small, so I guess a line is only reasonable option.

Have you seen https://wiki.openstreetmap.org/wiki/Tag:water%3Doxbow ?

Yes, but only area could be tagged as oxbow, not line. It is also designed for large and filled with water oxbows I suppose and I planned to restore existing parts of old historical 3km long river, but it’s most likely futile overkill, so I give up.

Yes, OSM is not the place to tag features that have been there but are no (obviously) visible anymore today. If the former oxbow is now a ditch with bushes you might want to check https://wiki.openstreetmap.org/wiki/Vegetation .