I was struggling to remove useless redirect from the wiki page, so the link would clearly show as red (page doesn’t exist). Example page Key:sanitary dump station:suction - OpenStreetMap Wiki
While I managed to remove the redirect, it seems like the page is still there with empty content. Is there any way to actually remove the page itself?
I guess, IIRC, this does the magic (after some time and manual review by someone)
{{delete|reason for deletion}}
as contents
You do realize I hope, that the intention of a red link is a request for someone to write that article. So there is always a chance that that will happen and de page will resurface. 
Yet another intention of red link - clear indication “there is no such page created as of now”. If the link is blue - it sends me a message “oh, there is a dedicated page about it”. And clicking it and getting redirect (especially to the very same page I was reading) is useless and not helpful.
Sure if someone one day writes a proper wiki page - it’s great, perfect! But if there is no page - let’s clearly say it 
I see that it is now marked for deletion - note that current policy is to wait at least few days in such case - before admin will delete the page
Not sure I agree, at least for documented tags.
For example, having redirects helps indicate valid values and puts a 
next to that value in Taginfo!
I disagree with this statement. Redirects in a wiki function as keys to a topic, whether that topic has its own dedicated article or just has a short section in an article about a wider topic. I must admit, however that the mention on the Tag:amenity=sanitary_dump_station - OpenStreetMap Wiki article is very, very minimal.
For this, in my opinion, the redirect should not be removed, but the link in the redirect’s target should be removed.
Keeping in mind that deleting the redirect will also break external links that are pointing to it. The chance for that increasing the longer the article/redirect exists. And this redirect exists since 2015.
chances that someone linked specifically to redirect seems really slim, and deletion log should contain mention of page that was targeted
TagInfo, iD and JOSM do but I suspect that those will automatically be removed if the article/redirect no longer exist.
not sure how JOSM links but taginfo link is definitely automatic, the same for iD one (if it even link wiki page rather than data item)
I can create short stub about the key.
feel free to do so!
Well, you shouldn’t really, because this key is less used (221 usages) than more used pump-out(511 usages)
No problem, let’s declare it as deprecated.
P. S. Widely used keys and tags should be documented. For deprecated keys and tags, documentation can help find alternatives.
I would be cautious about use of the term “deprecated” - there is a higher bar than a couple of forum posts for suitability of that label.
Instead, you could document it as “less common than xzy”.
I have no preference for one key over the other with sanitary_dump_station:pump-out vs. sanitary_dump_station:suction, and sanitary_dump_station:round_drain vs. sanitary_dump_station:gravity. However, I would appreciate a procedure that can be applied to other cases as well.
It should be noted that the latter keys (suction and gravity) were approved in 2015. In Germany, they are still the more widely used keys (suction 103, pump-out 7).
Is there anymore (dicussion / proposal / etc.) to prefer pump-out and round_drain and deprecate the other ones?
Purely from global usage you can see which one is more popular globally. And in any case, a few hundred of nodes is quite minor “usage” in the span of 10 years.
I would suggest just remapping to the more popular tab so we all can live easily.
If the (global) usage was that clearly. There is a single event in 2016 that boosted the use of pump-out
and one in 2021 that boosted the use of round_drain
If you subtract those events the increase of both alternative tag values is parallel and nearly the same. As I said, I don’t really care which of the tags are preferred. But due to the equal increase and the overall still low number of uses, it’s not clear or decided for me here. Therefore, I find it difficult to delete them or assign them the status of “deprecated”.
Looking at the difference between 2020 and 2022, the clear difference is the very large number of changes such as this one by https://www.openstreetmap.org/user/Niels%20Elgaard%20Larsen. The source was given there as “site website”.
and for completeness that was likely https://www.openstreetmap.org/changeset/42907258 by https://www.openstreetmap.org/user/fatuus. The source was “OpenStreetMap (Mapnik Black & White), open source”, which might just mean “a list I found on the internet”.