If your product stays current/modern with paid new features and the free portion is left to rot, it is enshitification.

I disagree. If your free version isn't changing, then it isn't enshitification. This is the freemium model. It gives basic functionality to everyone in hopes of converting them to (or using them to attract) paid users. That's not enshitification. That's giving someone a free sample.

Enshitification is not a specific behavior or feature set. It is a product lifecycle of first tailoring your product to the customers to increase market share, then tailoring to advertisers to increase revenue and solidify market dominance, then tailoring to investors and squeezing your customers and advertisers for every last penny and cutting costs wherever possible as your product becomes worse and worse for the original customers. It's the shifting of target audience and abandonment of the previous that is a natural byproduct of infinite growth.

This I agree with, I just don't think it is an accurate description of the situation above.