I think there's a difference between "all politicians are liars", and "several people currently in positions of power are repeated being untruthful with absolute impunity because of the system of politics that is enabling them to do so without consequences"
Which, now that I say it out loud, do sound very very similar.
But this feeling that they are all liars is not new.
In fact, at least with politicians they have a very easy "tell": You know when a politician is lying because their lips move.
(That joke has been around since before I was born, and I was born in the 80s.)
The difference isn't that the country thinks politicians are liars. That has not changed. The difference is that there has always been this "honour code" in politics, where if your sleaze was found out, you fell on your sword. But this was not ever backed up with any law with teeth, and so when the likes of Boris repeatedly lies, he can just throw another flunky under the bus to appease the crowds, or do something stupid to make the crowds laugh, and carry on as normal.
I don't think politics ever needed any outward credibility. It never had any. It needed external accountability. And without encouraging the media and the public to hold them to account when they do lie, we'll never get that accountability. In effect, the post-truth weakness you describe should never depend on public perception. In a well functioning government, repeated unrepentant deception should always trigger some form of legal jeopardy.