At some point OnePlus announced that they will stop sharing firmware blobs. Lineage os team announced that they will be dropping the support. Then after another few years they were back. I remember because I bought 3 and I was planning to stay with that brand because of easy unlock (via ADB), decent price and good Lineage support. Probably OnePlus reconsidered this at some point. Right now fairly new ones have support. Maybe OP was unlucky and bought one of those models from this period of time.

This has nothing to do with the unlocking though. Unlocking a OnePlus phone is just standard procedure and requires no involvement by the manufacturer.