These tags are all valid, but objects with these are not displayed on any map (neither Standard, CyclOSM, Tracestack Topo…). Examples would be:
natural=hot_spring (Tag:natural=hot_spring - OpenStreetMap Wiki)
historic=tomb (Tag:historic=tomb - OpenStreetMap Wiki)
historic=ruins (Tag:historic=ruins - OpenStreetMap Wiki)
and perhaps many others. Is this because they are still being developed?
There are definitely maps that show these. As an example, here’s a taginfo search for natural=hot_spring. It looks like OsmAnd shows those. historic=tomb has more projects supporting it - there’s one here. historic=ruins also has lots of support.
In my case, I did go through the usage of historic=ruins to see what features it was used on and what other tags it was used with - search for “ruins” in here. You can see that objects with this tagged get shown with different icons depending on how the tag is being used - it’s not as simple as just saying “render this tag”.
For historic=ruins, now I can see that it’s too general, so it should be used with other tags. However, natural=hot_spring and historic=tomb themselves are already quite descriptive, so I don’t see why OSM Standard does not support them?
What you call “OSM Standard” is Carto, one of the many rendering of the OSM database. There are a lot of things that are not rendered simply because the map would be too cluttered if everything was displayed to the final user. I suspect that there are a lot of other rendering that shows hot springs (maybe of these thinking focused maps). If it’s on Carto it doesn’t matter: if it’s in the database it will appear when necessary.
To add to Andy’s answer, even if your examples are rendered by some, there are definitely plenty of tags that aren’t used by any map renderer. Assuming a given tag means something and isn’t a typo or vandalism, it probably still has value. A data consumer of some kind may come along later and decide to start rendering or otherwise using it!
Given that currently natural=spring is rendered as a blue circle, I don’t see why rendering natural=hot_spring will clutter the map. Hot spring is clearly just a subcategory of spring. The blue circle can be slightly modified, adding two or three S-shaped lines above it to represent the vapor, to depict hot springs, or if the developers are too lazy then simply reuse the blue circle should be totally fine.
These are just nodes, not areas so they are not taking that much space to the point that the map becomes cluttered.