When I was at school 3 score years ago, spelling, punctuation and grammar (SPG) were drummed into us daily. Also my mother was a English teacher, and would read through my homework to make sure I'd got all the SPG correct.
When my kids, now in their 30s, were at school, it seemed it didn't matter any more. I once asked my daughter's primary teacher why she hadn't corrected my daughter's mistakes. "Oh, we have to let them express themselves, you know". Well thanks, but neither of them have ever caught up, and still don't know all the "rules" of SPG.
My husband is moderately dyslexic, so I do quite a lot of proof-reading for him, as he writes a lot of official reports. I've written him a guide to usage of apostrophes but he still doesn't get it!
I am not too bothered by errors in posts on Mudcat, but I do think it is important to get official documents for publication correct, and it does annoy me to see such publications littered with errors on occasion. Even designing our Folk Festival brochure, it can do with 3 pairs of eyes reading through it to spot any errors of SPG or fact, or inconsistencies of formatting.
And you can turn Apple's very annoying auto-correct predictive text off!