I'm ok with a version of this as a concept. The version being when a feature is technically present in every SKU, but requires an extra purchase to unlock. A reply to you specifically mentioned subscriptions, which I very much do not like (except in cases where the feature requires ongoing costs), but there are many cases where having every version contain the feature, but requiring a purchase to unlock it is pro-consumer, and is a win-win-win (or at wist a win-win-draw). It can, under the right circumstances, allow the product to be available for a cheaper cost than it would otherwise be. So people who are willing to pay for it are better off, people who aren't willing to pay for maintain the option to change their mind for a nominal fee, and the company probably makes slightly more profit.

All that being said, in my opinion, it needs to come with several features:

1. no subscriptions for something that is just a one time unlock 2. It needs to be legal and protected for customers to figure out how to unlock it on their own without purchasing the unlock.

I haven't thought enough to have a strong opinion on the exact situation you describe (where it is present and an unlock isn't available at any fee), other than to say I'd still argue strongly that customers figuring out how to unlock it on their own should be legally protected.