Would be great to get rid of slf4j but clearly a big step forward

The plan would be to allow some customization by the plugin later, so that they can add/remove packages and fine tune their class loader. In most cases, slf4j is not a problem.

In most cases, slf4j is not a problem.

Well, from my perspective we decided to use maven.api logging api and let the mojo use or not another API/binding/solution so there is no more reason to let slf4j leak (or then we should let leak other stuff too).
Slf4j was not a problem until it became and the choice was to give more guarantees to the mojo writers so default is probably better not having it and use tuning options to let it pass in unsafe mode.

But just my 2 cts indeed.

Resolve #9683