I mean exactly that it's not obsolete, because there is no standard CSS property that could have obsoleted it. (Even though every browser does have one; it's just vendor-prefixed.)

Ok, maybe this will make my point clearer:

    <center style="background-color: green">
    <div align=right style="width: 10ch">
    hello
    </div>
    </center>

To replicate this in standard CSS:

    <div style="background-color: red">
    <div style="text-align: center; margin: 0 auto 0; width: fit-content">
    <div style="text-align: right; width: 10ch">
    hello
    </div>
    </div>
    </div>

Or you can switch out of flow layout:

    <div style="background-color: red; display: grid; place-content: center">
    <div style="text-align: right; width: 10ch">
    hello
    </div>
    </div>

Ok, one semantic div less. But why can't I do this?

    <div style="block-align: center /* would act like text-align: -moz-center */">
    <div style="width: 10ch; block-align: right /* would act like text-align: -moz-right */">
    hello
    </div>
    </div>

If my browser understands the first variation, then getting it to understand the last one is in all likelihood not much effort; it's just substituting properties.

The only reason it doesn't, is that the property does not exist in the standard. So you have to use non-semantic centers instead of semantic divs, but that's frowned upon, so you have to use grid layout even though you aren't making a grid, just centered flow.

I just wish it were standardized, considering it's already implemented in all browsers (in the forms text-align: -moz/-webkit-center).