From Rodney Huddleston Geoffrey K Pullum. (2017). The Cambridge Grammar of the English Language. p.87.
For some speakers, irrealis were is not restricted to the modal remoteness constructions, but is found also in certain backshift and past time uses that bear some resemblance to them:
In summary, 'were'is both used in the modal remoteness constructions and backshift and past time uses.
To me, 'restricted' acts the same role as 'only', so omitting 'also' retain the same meaning.
For some speakers, irrealis were is not restricted to the modal remoteness constructions, but is found ['also' is deleted] in certain backshift and past time uses that bear some resemblance to them:
Is this analysis viable?