When objects close down, it's often better to re-tag them with a lifecycle prefix osm.wiki/Lifecycle_prefix than to delete them entirely. This way, any potential future features can re-use the old data, for example geometry or address/phone tags.

Anything that physically exists is allowed to be mapped, even if you're not allowed to use it. In these cases, consider access=no. I know it seems silly to try and maintain a routing network of inaccessible paths, but I could imagine for example an app that would find the nearest freeway refuge in emergencies.

Nice!

You can map traffic signs, including speed limits: osm.wiki/Key:traffic_sign and I don't think it's redundant to do so: the sign is a physical object that can be mapped, and the limit is an attribute of the road or rail.

Do I understand right, that the fixme asked how to tag something, and because there is no tag for it currently, you simply removed the fixme? If a tag for it is made in the future, that will make this hard to find.

In the absence of separate sidewalk ways, however, it is necessary to connect them to maintain the pedestrian routing network.

Many of these businesses that you've added were already mapped as part of the buildings. Please check that you don't add duplicate data.

Just because it isn't typical doesn't mean it's wrong.

It is tagged surface=dirt, and it should not be tagged natural=sand. Neither should SF Giants stadium.

"This tag should not be used for the infields of baseball or softball fields (a form of tagging for the renderer) - use surface=dirt instead." - osm.wiki/Tag:natural%3Dsand

Thanks. I changed the outer part to a building outline and filled in the rest of the building parts: osm.org/changeset/137765483

It should still have some sort of feature tag describing what it is though, a node with nothing but name=* isn't very useful. Maybe some other value for place=*, like "quarter" or "neighborhood"?

Thanks for contributing to OSM! Please use the changeset description field to describe the changes you made and why; for example, "Changed the website of a post office to be up-to-date." There's more guidance and examples at this link: osm.wiki/Good_changeset_comments

I don't understand what you mean by "self relation" or relating objects "to" others. Relations are objects themselves, with tags, that contain other objects. Each child of a relation has a role. A building relation is supposed to have all of the tags for the building, and have all of the building parts as children, with the role "part." A building relation is also supposed to have a building way with the role "outline," and that seems to have been the only error.

I would be very cautious of blindly fixing validator issues. They often aren't causing any harm, but removing data rather than fixing it definitely does.

See here for documentation on undoing changes: osm.wiki/Change_rollback There's a tool for it in JOSM, as well as two web apps.

See here for documentation on the building relation type: osm.wiki/Relation:building It may have been necessary for the 3D mapping to work, though it looks like it was missing an outline.

Please use descriptive changeset messages. For example, "Extended a residential area." This page has more guidance and examples: osm.wiki/Good_changeset_comments

It's still useful to keep address locations, even if there is no occupant at that address, so that geocoding still works.

Well, the "easy" fix is to move the descriptions into description=. There's also sport=billiards, but that usually needs a primary feature tag. In general though, you can always ask the tagging mailing list, OSMUS Slack "tagging" channel, or the forum, for tag suggestions.