Bitcoin or crypto-backed loans are nothing new. They're offered by both crypto and traditional financial companies. Even in this forum, it's available. A crypto-backed mortgage is just another kind of crypto-backed loan.
But why do you think it's a smart move to use your Bitcoin or crypto as a collateral when the market is down? Although, yeah, it helps you avoid getting liquidated, it also means you're getting a much lower value of what you have. You're risking your Bitcoin to a third-party in exchange for, say, 40% or 50% or 60% of its value. Personally, I don't think it's worth it.
Like you correctly said avoiding to getting liquidated is the main reason I find it interesting. Well this might now be worth it but also selling of your bitcoin for real estate in my opinion is worse especially when you are holding bitcoin that can easily go parabolic at any time, also I find it interesting compared to the traditional means because it offers a less restricted real estate opportunity.