Data preloading expiration and opt-in/out per route

Describe the problem

Data preloading is often enabled site-wide by including data-sveltekit-preload-data="hover" in app.html. This can then be disabled/enabled per-section/link, which is not ideal. Also, I am not quite sure if preloaded data ever expires.

I have a data-intensive dashboard page, on which data can get outdated quite fast (a few minutes), so I don't want pre-loading on it. This is linked in multiple places, so I had to manually add data-sveltekit-preload-data="none" on every link to this page, making sure not to forget any, also remembering to disable preloading if I later add another link to this page.

The main issue, however, is that, once hovered and the data is loaded, it never seems to expire. So, if you use the default preload-on-hover behaviour, and you follow these steps:

1. Hover a link
2. Wait 5 minutes doing other stuff on the current page
3. Go to the link

The data will be outdated (from 5 minutes ago).

Describe the proposed solution

Some pages don't do well with preloading, but this is per-page, not per-link. It makes more sense to say "this page can get outdated fast" rather to specify per-link (why would a link to page X need preloading but a different link to the same page X not need it?). There should be a export const dataPreload = false in +page.ts to prevent data preloading for a specific page.

As for the main issue, there should be a way to tell SvelteKit to not hold onto preloaded data for too long.

Importance

nice to have

Start by reviewing the site-wide setting in app.html, the proposed +page.ts export, and links using data-sveltekit-preload-data. Determine how per-page opt-out and expiration should behave for preloaded data, then verify the hover, wait, and navigation scenario described in the issue.