> Leaning on a proprietary solution also means you're downstream of any decision the makers of the tool make, including choosing to nuke your (and/or your business's) use case.
My understanding is that OP is advocating free/libre software protects a business from changes in software that could affect the business' use case.
This risk is present in both proprietary and free/libre software. Maintainers may remove features that some users find critical. If a feature is removed and your business requires the feature, you now need to maintain a fork of the software or contribute development time upstream.
Depending on the project, it may move slow enough for the dependency to be slow to update and it be fine. Depends on what you're doing.
At least in the case you mentioned, I can revert to an older version and freeze it there until my business can sort out the way forward.
Ideally, one chooses dependencies that are easy to replace and reasonable to maintain for a bit, if needed. Or if you're lucky, no dependencies at all!
I can't comment on the LDAP thing on a technical level, but if the focus is on libre software, why is it apropos to list proprietary things that it's compatible with? Software that boasts this compatibility doesn't always keep it, or sometimes loses it due to deliberate action from the proprietary party. I don't think I'd list Active Directory either. It's not something one can vouch for unless they're literally Microsoft.