I'll expand on Lazy Badger's answer with some references.
The SVN Group refers to this feature as "obliterate". Knowing this helps us dig up some more information about it.
To start with, here is the SVN group's JIRA issue about supporting obliteration. It was opened in 2001 (!) and has generated a lot of discussion both for and against implementing it. The last comment by a project committer on this issue indicates he made some progress on it, and also points us towards a milestones/feature status page.
The development notes for this feature have not been touched in some time:
>svn ls http://svn.apache.org/repos/asf/subversion/trunk/notes/obliterate/ -v
1398429 stefan2 Oct 15 2012 ./
878962 julianfo 872 Aug 20 2009 README
952192 rhuijben 5175 Jun 07 2010 comparison.txt
1398429 stefan2 2488 Oct 15 2012 design-audit.html
1398429 stefan2 3839 Oct 15 2012 design-authz.html
1398429 stefan2 18664 Oct 15 2012 design-repos.html
1398429 stefan2 2500 Oct 15 2012 design-wc.html
952192 rhuijben Jun 07 2010 fspec-cc1/
883799 julianfo Nov 24 2009 fspec-dd1/
952192 rhuijben 7742 Jun 07 2010 high-level-design.txt
904301 neels Jan 28 2010 hooks/
876577 hwright 39780 Mar 12 2009 obliterate-functional-spec.txt
1398429 stefan2 7541 Oct 15 2012 plan-milestones.html
880336 julianfo Oct 28 2009 presentations/
952192 rhuijben 11266 Jun 07 2010 req-spec.txt
887471 julianfo 290129 Dec 04 2009 schema-bdb-dd1-after.svg
887471 julianfo 297789 Dec 04 2009 schema-bdb-dd1-before.svg
Note: The last "real" commit was r906520 (Feb 4 2010); others after that were repository maintenance-related.
On the developer's mailing list, the word "obliterate" hasn't been uttered since 2014, and that was a parenthetical reference to it as an external tool:
It is perfectly reasonable to expect repair, analysis and offline
filtering (obliterate) tools to access FSFS on-disk data.
Clunky work-arounds do exist, primarily revolving around the use of svn-dumpfilter. Though for most, dumping and importing the repository for something that could happen often is a non-starter.
Because it is on the roadmap, I wouldn't go as far to say that it will never be implemented. But, because (as Lazy Badger mentioned) permanent deletion of content flies in the face of Subversion's original design and purpose, it's implementation could be a long and difficult process.
Please keep in mind the standard OSS disclaimer: this project is mostly made up of individuals who contribute to it in their spare time. There are other features/fixes that have higher priority than this, so if you'd like to change that, let the developers know.