Can we deprecate theallowEmptyString option instead? If someone wanted to disallow empty strings, they could use theNotBlank validator instead, or am I missing something?

Good idea, this was actually suggested when the option was introduced in 4.4 (see#31528):

In my opinion, the flag could even be removed later.

Thank you@xabbuh.

Just wondering how we can achieve the "old behaviour" now when combining min, max and allowEmptyString=true

It used to be "ok" to have have either an empty string OR apply min-max-constraints - this is gone now. Or am I missing something?

Precisely for such use cases you can combine theLength constraint with other constraints usingAtLeastOne (see the example in the upgrade guide):https://symfony.com/doc/current/reference/constraints/AtLeastOneOf.html