During heavy market volatility, do brokerages modify margin requirements across all accounts? Or is it on a case-by-case basis?

Currently I hold 100% allocation of BOXX ETF, which is considered 'ultra low risk'. My brokerage Vanguard offers a 50% LTV in the form of margin cash against my equity in BOXX.

During heavy market volatility, such as the 2008 financial crisis or 2020 COVID crash, do brokerages like Vanguard make sweeping company-wide adjustments to LTV across all brokerage accounts? For example, reducing the maximum LTV on all margin enabled accounts from 50% to 35% regardless of the performance of the securities backing the margin?

Or historically have brokerages like Vanguard changed margin terms on an 'account by account' basis during a market crash? For example, if someone's entire portfolio is in a position like BOXX which is ultra low risk and does not drop in price during high volatility, they're margin terms remain unaffected at 50% LTV for margin cash (as it was prior to the volatility). Versus another investor's account that has a 40% drop in a higher risk position used to secure the margin cash would have their margin terms reduced from 50% LTV to 35% LTV, for example?

I spoke to a Vanguard representative over the phone and they suggested it's more of a case by case basis, but I wanted to confirm if anyone here has had any different experiences.