Besides, it would be nice to be able to set global options for the slugger (i.e. at https://symfony.com/doc/current/reference/configuration/framework.html) - just like cocur/slugify allows: https://github.com/cocur/slugify#symfony-configuration

Besides, it would be nice to be able to set global options for the slugger (i.e. at https://symfony.com/doc/current/reference/configuration/framework.html)

Given the reason why you wanted to introduce this method I think it would make more sense to introduce that possibility than adding the setter method.

Yep, this is intentional: having the service immutable is desired.
I guess it's possible to wire a new value in a compiler pass, or by replacing the service definition, isn't it?
Dunno if this is common enough to be added to configuration though.
To be discussed for 6.1.

Setter vs. configuration: Well, both would be nice ;-) If you're using the slugger in 100 services, you need the configuration; if you have one exception, you need the setter ;-)

Version: I didn't really mean to get this into 5.4 ;-)

This is the default $symbolsMap:

'en' => ['@' => 'at', '&' => 'and'],

... and this looks like a "template" to me, showcasing how to set it up. So at least an easy way to get '&' => 'and' in more languages would be nice :-)

This is the first Symfony feature I'm seeing where the framework integration is harder to use than the standalone (cause you can't just pass it to the constructor). Shouldn't this be the other way round?
But maybe the following is the answer and it's just not documented:

"wire your own configuration" or "replacing the service definition": How does this work?

if you have one exception, you need the setter ;-)

No. You need a separate instance. If you use the setter for this exception, you also affect the usages done in other services after you called that setter.
That's the main reason why we avoid setters on services. It makes things a lot harder to reason about.

Should we close this one?

Indeed, if I pass ->args() in my services.php, it works:

$services->set('slugger', AsciiSlugger::class)->args(['de', ['de' => ['&' => 'und']]]);

However, if I pass ->arg() it does not work:

$services->set('slugger', AsciiSlugger::class)->arg('$symbolsMap', ['de' => ['&' => 'und']]);

Does anybody have an idea, why?
The argument name $symbolsMap is correct, see https://github.com/symfony/symfony/blob/6.2/src/Symfony/Component/String/Slugger/AsciiSlugger.php