I asked the same exact thing in Feedback Assistant 4 years ago, and they told me that they have no plans on making old macOS installers available. Apparently they have no interest in allowing developers to support older macOS systems, which, assuming that their main interest is maximizing profit from Mac sales by pushing people to abandon old Macs that don't support the software they need anymore, totally makes sense.
Topic:
Developer Tools & Services
SubTopic:
Xcode
Tags: