Since it's painless/cost free, with zero consequences going forward, I opted for defaulting it on.
But I was unsure, hence the question.

The concern is, if they leave off, then sometime down the line, when they try to use keys built on a new platform on a legacy platform, they'll then have to rebuild the keys with this option on.

Whereas before this change, the max default was set into to the key, meaning <4k key+payload keys automatically worked with legacy systems.