Thanks @Brecht Van Lommel (brecht), committed as:

• rB4510f88d00a7
• rBb7d942446f8a

As mentioned in the commits (and here) if we agree on I gladly change how OUTLINER_OT_unhide_all works.

@Dalai Felinto (dfelinto) I think we had this conversation before during the code quest. Here's my feedback on the topic:

All the time in edit mode I select and hide geometry as a quick way of saving a selection for later without it being in the way of me selecting even more. Once I have everything selected that I need I hit Alt + H and I have my full selection. Anything that will be hidden was first selected so it makes sense when it's shown again that it is still selected. And this behaviour is extremely useful, even in object mode. It's even possible in other modes like pose mode.
I don't like the behaviour that an object that is hidden and then show again becomes stays deselected. It's inconsistent with the other modes and makes the use case of what I described above much harder.
The workaround for me with the behaviour of this commit would be to temporarily create a collection to link the selected objects into and hiding the collection ... or just never hide something again if I want to keep it selected.
This is extremely slow in comparison though.

Keeping it the way it worked before this commit and how it worked in 2.79 is just plain more useful. This commit just gives the user no advantage whatsoever so why change it?

Hi @Julien Kaspar (JulienKaspar) I clearly expressed myself poorly. I agree with you and this is the exact behaviour that I implemented. But to be sure, please try a build after said commits, to see if this is what you had in mind.

@Dalai Felinto (dfelinto) Ah ok. The latest build I downloaded at home clearly wasn't the most up to date. As far as I can see it's now working just as I expected it to.