I concede that it is not precisely OSS. But if I tell someone that it is source-available, they will expect some kind of license restriction for any use. If I tell someone OSS, they will expect mostly what the Sentry license entails, unless they are a competitor, in which case I really don't care what they think.

I wish there were a popular term that conveys exactly how Sentry license works. But, there isn't - so I think it's fair to say open source, maybe as a general term. I'll change it from OSS to open source