I think the 'greengrocers' apostrophe' is nothing very new.
IMO many people think there some great mystery to where to put an apostrophe, and it's going to be too difficult for them to fathom - that is if they even realise that they have a problem with it, which IMO many don't.
Why it's evidently so often badly taught I don't understand. There's really nothing difficult about it. I used to teach English to mostly Arabic speakers and they would grasp it pretty quickly if presented clearly enough.
If it really is the case that some teachers don't know the rules, because they themselves weren't taught, there are cheap enough books or the Internet to teach them. Plus, IMO all trainee teachers, should be tested on basics like this, and sent for remedial English if necessary.
It appals me when people say these things don't matter any more. They most certainly do, and e.g. basic mistakes on a CV or covering letter can wreck a young person's chance of a job, if they're competing with a lot of others with much the same qualifications.
I know of one HR officer, my dd's generation, not a pernickety old,pedant like me! - whose method of weeding out a mass of very similar CVs was to bin any with basic SPAG mistakes.
And no, you can't rely on spellcheck.