Netsurf Cache

After an informative thread on how the disc cache is used by Netsurf I
wrote:

On 13 Apr 2016 in article <55705bbd12netsurf@avisoft.f9.co.uk>,
Martin Avison <netsurf@avisoft.f9.co.uk> wrote:
> I have now deleted the whole cache, and reset Netsurf to the default
> values for disc of 1024MB and 28 days, and memory is set to 50MB.

> Time will tell what the effects of this are!

As 14 months have passed, I though I would report back. The current
contents of my RISC OS Netsurf v3.6 disc cache is...

Total Files 5,204 Size 530,576,177
Expired Files 4,773 Size 404,997,322 before 30 May 17
3xExpired Files 3,337 Size 280,195,992 before 07 Mar 17
6xExpired Files 1,692 Size 151,695,867 before 20 Sep 16
Total Directories 19,648 Size 40,245,248 Empty 899

There are nearly 900 directories with no files in them - but that is
quite a small proportion of the 19,000 directories. And I know from the
old thread that these could be deleted without causing any problem. It
does still strike me as silly that the number of directories is nearly 4
times the number of files held in them.

However, I was surprised by the 90% files that were older than the expiry
date - indeed 60% were over 3 times the expiry age, and 30% over 6 times.
After more investigation, I could see that there are files right back to
April 2016 when the cache was initialised.

Are age expired cache files *ever* deleted?
If so, when, or are they only deleted if the size limit is exceeded?

Have any changes since 3.6 affected this?

In an earlier post in that thread I had asked...
> When are cached files deleted to meet the configured size & expiry?

and the reply was ...

On 13 Apr in article <20160413104528.GA24798@kyllikki.org>,
Vincent Sanders <vince@netsurf-browser.org> wrote:
> The cache is pruned only when adding a new entry which causes the
> overall cache usage to exceed the set level. at that point the least
> "valuable" objects are discarded until the size drops below the
> desired size. This process is subject to 10% hysteresis to avoid
> excessive thrashing.

I can understand how this applies to the maximum size, but it does not
seem to cover the age expiry.

Can expired files be manually deleted without causing problems?
I suspect that various cache index files would cause problems as they
would not be updated.

Thanks
Martin