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 securitie