Someone wrote an answer here that was interesting, and then deleted it quite quickly after a skeptical comment was added. But I think it forms the basis of a good answer to this question:
The Cambridge dictionary defines "pc":
UK written abbreviation for percent
And there are some other references to "pc" being British. However, I'm British (nationally and culturally) and I found it pretty odd, so it's clearly not that British. And, as noted by Edwin Ashworth, the Guardian style guide recommends "%":
% in headlines and copy
The Wikipedia article on "percentage" mentions:
although the abbreviations pct., pct, and sometimes pc are also used.
And points to a Telegraph article as evidence. So it's clear the Telegraph is one of the main places doing this.
The Telegraph style guide says:
Percentages: per cent does not take a full point. Use pc only in headlines and % only in tables. In City page copy pc is acceptable. All percentages written in numbers, not in words: 5 per cent, never five per cent.
It doesn't explain what "City page copy" means, but I suspect it means coverage of the London-based financial sector, so maybe it's a tradition there.
The Wikipedia article on "Percent sign" has a section on "evolution", which mentions:
At some point, a scribe used the abbreviation "pc" with a tiny loop or circle (depicting the ending -o used in Italian ordinals, as in primo, secondo, etc.; it is analogous to the English "-th" as in "25th").
The "pc" with a loop eventually evolved into a horizontal fraction sign by 1650 (see below for an example in a 1684 text) and thereafter lost the "per".
Overall, it seems like "pc" might well simply be a particularly archaic and British formulation, used in some circles with strong traditionalist attitudes (although not all that much in British society at large).
What I thought was an increase in general usage might have just been that the algorithms that serve me news articles have been showing me more from the Telegraph or similar outlets than they used to.