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Google is preparing to fully remove support for non-A/B updates from Android, leaving A/B updates as the only officially supported OTA update mechanism moving forward.
However, this won't stop certain device makers from continuing to use non-A/B as they can either use their own implementation or just revert Google's changes. Google would need to actually mandate the use of A/B through the CDD or GMS licensing, which they've tried to do but ended up reneging.
How many devices are that old but still getting updates?
My 6 year old OP6 has A/B partitions.