Discussion: Deprecating <amp-img>

# Context
[`amp-img`](https://amp.dev/documentation/components/amp-img/) allows the runtime to effectively manage image resources based on the viewport position, system resources, connection bandwidth, or other factors. With increasing browser support for more granular control on the native `img` tag, it is time to discuss a deprecation strategy for `amp-img`.

# What would it take to deprecate amp-img?
To support a comparable feature set without `amp-img`, we first need broader support for [lazy loading](https://caniuse.com/#feat=loading-lazy-attr) and [aspect ratio](https://github.com/WICG/intrinsicsize-attribute), but there may be other critical features preventing the possibility of any imminent change. First steps are: (1) understanding the minimal range of critical features and (2) tracking ongoing progress for these across browsers.

# Open questions

- **Using the AMP cache:** How much can be delegated as a serving optimization?
- **Providing placeholders and fallbacks:** How do we replace placeholders and fallbacks?
- **Loaders:** Is there still a way to provide [default loaders](https://github.com/ampproject/amphtml/issues/20237)?

Start with the linked amp-img documentation and review the lazy-loading, aspect-ratio, AMP cache, placeholder, fallback, and default-loader references. Done means identifying the minimal critical feature set, tracking browser support, and resolving the open questions well enough to document a deprecation strategy.

Written by the indexing model from the issue text.