Do you know that as fact or is that pure conjecture?
As someone who’s been in education for just under 30 years I can tell you that grammar and a robust teaching of it has been part of the revamped syllabus for a number of years(September 2013). Tests of this begin as early as year 1 with a SATS test at year 2 and 6. There has been much criticism of the implementation of this with critics saying that it’s too much and children get bogged down in dogma.Simple rules such as “their, there and they’re” have been taught for generations. The distinction between the use of “them and those” still incurs problems in use most likely through the fossilisation of the wrong use by various communities over many years.
Grammar schools or selectives still exist but as you say few and far between. The National curriculum guidelines still have to be followed though those kids by the selection process will have learned the correct grammar usage to obtain a place in the first place. As an example, I did gain a place at a notable selective but correct use of English was drummed into me from and early age.
Based on experience of myself and others, as well as working in education.
I suppose it could also be how we define “grammar”. I was always taught to speak properly, but we were never taught the names of tenses or a technical way to structure a sentence. 16-year-old me would not be able to identify the adverb and adjective in a sentence, or say what a past participle is. Maybe I was just thick, but I got a “B” in English.
Perhaps the syllabi have changed in schools now, I did leave school a long time ago, but I’m sure I read an article recently that was arguing for schools to start teaching English grammar in order to make language learning easier.
I’ve learnt a couple of languages in my adulthood and I had to learn English grammar first, as the foreign languages were all explained technically.