I don't know enough about teaching in schools to comment on the current quality of teacher recruitment and ability. I have no complaints about my own children's teachers and think they do a great job.
I think good grammar and spelling is important in most contexts. The meaning of a sentence can be altered substantially without it. The ability to express a concept clearly, without ambiguity, is essential to the advancement of nearly everything. We cannot progress if we spend all our time having to clarify things first.
I also think that reasonable adjustments should not equate to a drop in standards. Once reasonable adjustments have been put in place, an individual benefitting from them should be able to perform at the same level of competency required as someone not in need of additional support.
That said, it's 2022. We have a wide range of technology available to us and we know that diversity in teaching methods maximises educational potential among students. Someone who is a gifted teacher but poor at SPAG should be supported to teach, with help for SPAG-related functions (even beyond reasonable adjustments if they're that good, such as having a competent secretary or paying for dictation software and a proof-reader). The trouble is no one wants to pay extra for that. I would, but I suspect I'm in a minority.
My last role required me to authorise files used in a legal context. I have definitely noticed a decline in ability to write coherently among new, generally young, staff and spent a lot of time coaching them. Most improved rapidly (make of that what you will) but it was rather concerning given that the files had the potential to fundamentally impact on someone's life.
Clarity matters. In teaching fundamentally so. However, I don't think dyslexia means you can't teach; far from it. Managing diversity means harnessing different strengths to overcome this. The trouble with a lot of state-based institutions is that they're only doing half the task and not adequately supporting their staff.