PS: That change is also missing in the upgrade guide. https://tailwindcss.com/docs/upgrade-guide

@obecker outline in v4 does that:

/* Specificity: (0, 1, 0) */
.outline {
  outline-style: var(--tw-outline-style);
  outline-width: 1px;
}
@property --tw-outline-style {
  syntax: "*";
  inherits: false;
  initial-value: solid;
}

Since the property has an initial value of solid it also sets the outline style already, so rewriting it to outline-solid feels wrong.

The one real change is that it now has a width of 1px instead of 1.5px (browser default), so you could argue that it should translate to outline-[1.5px] (although I am not even sure if that was the same in all browsers?) but the real issue is that in v3 you had to use both utilities like this:

<div class="m-10 outline outline-1">outline-1</div>

Where as in v4 you'd only use one anymore:

<div class="m-10 outline-1">outline-1</div>

If we renamed outline to outline-[1.5rem] that would order after the outline-1 utility so it would take precedence which also doesn't make sense.

I think that's something we probably can't automatically migrate but you're right we should mention that change in the upgrade guide, let me try to add something.