So, Sentry is no longer Open Source at all, though people might potentially be able to collaborate on three-year-old versions (or, hopefully, use whatever unified fork pops up from the last Open Source release).

That's disappointing. Unfortunately, the more companies use such licenses, the more others see it as "safety in numbers" for them to do the same.

I somewhat suspect companies that apply "creative" licenses like this haven't been under a corporate legal department before.

Using a standard license is great. Using a custom license that is extremely similar to an existing license is not as good, but is usually workable...

Showing legal how to use Github so they can confirm the age of the software you're using, then convincing them Github is a reputable party will likely result in an unpleasant conversation at best.

A conversation made far less pleasant by the ability to feed arbitrary dates into a Git repo's commit history: https://github.com/gelstudios/gitfiti

I'm sure there's a way to find the actual time a push happened; hopefully it doesn't involve subpoenaing GitHub (or Microsoft, I guess) for logs.