I think this feature would be quite useful. I am currently experimenting on spinning up a wikibase instance based on "rules" from a git repo and really would like to be able to start over every now and then. This should be possible via an api call and it also should reset the id counters. For self-hosted wikibase instances there is the following guide: https://www.wikibase.consulting/deleting-all-wikibase-items-and-or-properties/. I have not tried it but it seems to indicate some demand.

In the refinement session we talked at length about this issue. I brought up that I saw a possible conflict in providing the reset ability with wikibase.cloud wiki's being full members of the Linked Open Data web.

If we allow people to wipe their whole wiki, without even trying to use the Wikibase tooling to show what happened it could be hard for people to feel comfortable linking to these Wikibases. I was proposing perhaps some alternative solutions such as allowing people to delete their entities but not reset the counters. This may inconvenience the Wiki owner since from that point they may be then next creating Q12432 rather than Q1 which could be jarring.

@Charlie_WMDE was making the point that if the user wants to delete this data then is probably experimental and therefore other people shouldn't be linked to it/relying on it anyway. She was pointing out that for the owner who wants their Wikibase to be reset this seems like an unfair burden to carry given that they just thought that were were experimenting and perhaps didn't realise that they were forever "losing" Q1 or P1 of that specific domain.

I then failed to go on to say that this might then be a user education issue and that it if we were to offer this feature as described we might want to make it clear to the wikibase owner how they might be inconveniencing the community if other people were relying on that data.

I also suggested, as we have discussed before, that perhaps we could implement a one time amnesty for people who really don't want to lose their domain but do want to do this kind of "hard" reset.

A few thoughts from my side,
Defining "Emptying" would be useful here, does this mean just entities? users and wikitext pages as well? Other stuff too? Which one of these is actually most useful for users?

We should also consider at least just allowing users to reuse their own domains, this is how things would be "outside of wikibase.cloud", folks would be free to setup a wikibase on a domain, trash it and start again.
Originally disallowing domain name reuse was mainly to stop other people taking over domains of other deleted wikibases. IE, User A makes foo.cloud, then deletes it, then User B makes it "hijacking" the name.
I agree stating why this may not be wanted in some cases makes sense, but also I can see why creating and trashing a wikibase 10 times in a day when you first set it up sounds fine, even with the data reuse topic flagged up.

Some middle ground between ideas would perhaps be to have a button, that after 1 week, or after 1 month, or maybe just right away warns the user with why they might not want to delete everything, OR why they might not want to reuse a domain, depending on emptying, or reusing would be decided as the path forward.

Generally I'd compare this to something like the github or wordpress platform.
Both platforms allow you to reuse domains (repositories or domains) within a user.
Github then has a rule that reads something like for users "Once you delete, that name can never be used again for security reasons." (similar to not allowing users to use each others domains)

Yeah, I think I agree in general that there's a careful balance to be struck.

To provide another external comparison though: npm - this doesn't
allow deletion after a certain (rather short) period of time. I would
suggest this is because people specifically depend on the names;
whereas in an ideal world git is meant to be a distributed store.

Also git will inherently warn you if you try to "pull" from a data
source that is not following a similar historical path to your data.
This versioning system isn't there for an LOD client like another
blazegraph instance pointing at at this query service so the
"dependency user" would just see any of their federated queries break.

Given that users can currently make and delete an unlimited number of
wikibases we should really be nudging them towards this if they want
to constantly create and then trash a wikibase rather than reuse.

tl;dr I think we should probably allow users to do this but we need to
make it clear to them why, in general, this isn't a great idea and is
sort of "selfish" if other people are (or they hope people will) be
depending on their data and a better alternative would be to create
some temporary test wikibases.

After my experience trying to learn how to make real use of Wikibase I find the need to destroy instances, specially to restart the items/properties numbering. Today I found I can't reuse a subdomain name of a destroyed instance so, now I think it would be helpful to blank/reset an instance. For example, in our current project we'll have a lot of testing to do.

So, I'd welcome a simple feature for the complete reset of an instance.

Thanks.