Why not? You can modify/redistribute it, you can't just deploy it to production without having a valid subscription. It's not Free Software, but it is Open Source.
Free Software and Open Source's differences are more philosophical than technical/legal [0]. This means that if something cannot be Free Software it cannot, more often than not, be Open Source either.
If you cannot deploy it without a paid subscription, it's not Open Source by most commonly accepted definitions of the term.
It's not Open Source because the terms aren't compliant with the OSD[1]. And while the OSD may not be a de-jure definition of "Open Source" it is the de-facto definition in the popular vernacular around this subject.
If you cannot deploy it without a paid subscription, it's not Open Source by most commonly accepted definitions of the term.
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[0] https://www.gnu.org/philosophy/free-software-for-freedom.en....