These are a nice way to show larger companies you want to work with that you've made long term considerations for the sustainability of your software. We've brought this sort of thing up later in deal cycles if prospects have concerns over company future. We have contract clauses ready if they want to participate in the code escrow and many are happy to pay to join on as a beneficiary if we shut doors or discontinue our product.

In practice though, code escrows are kind of fraught with issues -- deposited source code that doesn't compile, out of date, etc. It's up to vendors to deposit working materials, as there isn't usually any kind of audit process. I can't imagine the costs and time it would take to pick up a vendor's source code, staff some maintainers, and get them efficient. Enterprise customers are usually shopping by requirements, not underlying tech, so they are likely not even aware of the tech they would need to hire for.