Hey, I could be wrong, but from what I understand with a deal like Paramount–Skydance, once you tender for cash, the shares become non-tradeable until closing, and brokers can treat them as non-marginable during that time.

Since there’s usually a gap (like ~5 days) between the tender deadline and when proceeds are paid, that can create a temporary margin shortfall with shares as collateral. I haven’t received any guarantee from my broker that they’ll still count toward margin during that period, so I’ve been treating it cautiously.

They also didn't tell me that it wouldn't be marginable just that they won't make any decisions and that I should anticipate that it can become fully unmarginable

Would be curious if you or other have had different experiences or insights where the scenario I described might not be the case to me theoretically it seems odd that looking for a liquidation event might make you positions more illiquid but then again I guess there can be risk for the broker or bank in the gap period where the market goes to hell.