It should be noted that really it was the collapse of the t-v distinction (formality) rather than the singular-plural distinction, it's just that it's a less relatable to the modern era thing to say that it was a fight over if "thou" has a place or if "you" should once again encroach upon its role and take the role of the singular informal tense as well. Long story short the Quakers refused to use the formal because all people should treat all people as friends and anti Quaker sentiment contributed to the death of "thou". Because nothing pisses people off like being told everyone should treat everyone well.

Here's an article about it

Another fun bit of 18th century culture wars involving the Quakers is The Public Universal Friend, a nonbinary preacher in late 18th century new England. It went better than you'd think and worse than you'd hope.