Last year I had a KS3 tutor group. They had a geography teacher who left at Christmas due to ill health and could not be replaced. We were short of science teachers and about 50% of them did not have a qualified science teacher for all their lessons. We lost some maths teachers through the year, so progressively more of them spent lessons without a qualified maths teacher as well.
Due to all of this chaos, there were a lot of teachers taking more time off sick, as well- by the start of the summer term, some of them were telling me they regularly only had 1 "normal" teacher a day, or sometimes a full day of supply/cover work.
As you say, the learning is compromised, and that, combined with lockdown, means they'll go into their GCSEs with massive gaps. I'm no longer working at that school, but my understanding is the situation with science has got worse, and there are still issues with maths too- so some of them will go through this year without permanent teachers in those subjects.
The school has changed its timetables to try and protect Y10 and 11 in science. This limits the option to do triple science, but that's a separate issue. Even with the timetable changes in place, most of them will get to the end of Y11, having never been taught by a physics specialist. Many of them will come out with a weaker science grade than they might have hoped for.
A 5 instead of a 6 means maybe they can't do science A-levels. A 6 instead of a 7 maybe limits the uni courses they can apply to. A 3 instead of a 4 may mean they have to do a level 2 course at college, instead of level 3.
But I think it's such a slow drip, that people don't always see the impact until it is far too late!