Change 364028 had a related patch set uploaded (by Golopotw; owner: Golopotw):
[mediawiki/skins/Vector@master] Drop position:absolute of logo.

https://gerrit.wikimedia.org/r/364028

Change 364934 had a related patch set uploaded (by Krinkle; owner: Krinkle):
[mediawiki/skins/MonoBook@master] Remove "position: absolute" and z-index from #p-logo

https://gerrit.wikimedia.org/r/364934

Tested applied styles in IE 7 (identical rendering to before):

Change 364028 merged by jenkins-bot:
[mediawiki/skins/Vector@master] Remove position: absolute from #p-logo element

https://gerrit.wikimedia.org/r/364028

Change 364934 merged by jenkins-bot:
[mediawiki/skins/MonoBook@master] Remove position: absolute and z-index from #p-logo

https://gerrit.wikimedia.org/r/364934

On T171195, @zhuyifei1999 wrote

The z-index removal may have caused div#column-content to cover #p-logo.

Indeed. The margin-left for .mw-body on div#content.mw-body is reserving all transparent space on the left of the content area, thus preventing the logo from being a click target.

What I don't understand is... why does this only affects the logo anchor, and not the regular anchor links in the sidebar?

I've created an isolated test case here:
https://codepen.io/Krinkle/pen/pwMKVe?editors=1100

In T171195#3457477, @Volker_E wrote:

First idea without intense testing: Using position: relative; instead with z-index.

Indeed. This appears to fix it, but I don't quite know why.

Seems fixed in Vector.

In T170053#3457897, Legoktm wrote:

This was reverted in MonoBook, see T171195: Site logo is no longer clickable in monobook.

@Golopotw: Hi! This task has been assigned to you a while ago. Could you maybe share an update? Do you still plan to work on this task? Thanks! :)

@Aklapper: Hi! I think it will be better to reassign this ticket to another developer. Thanks for the invitation!