It looks like we can't remove deprecation in Symfony 5 without dropping support for Symfony 3.4
Considering that 3.4 is going to be unsupported next November, I would drop it. But I'm open to suggestions.

Hello,
For a start, thank you for the contribution.
I temporarily fix this issue setting SYMFONY_DEPRECATIONS_HELPER env var to weak (see https://github.com/KnpLabs/KnpSnappyBundle/pull/268/files#r467055035) on all the test cases but this change should be reverted as soon as we remove the support for Symfony 3.4.
I'm in favor of removing the support for Symfony 3.4, if you want I can deal with that in the coming days.

Thanks @alexpozzi
I was just worried about tests failing in this PR.
Do you think it's better, for now, to remove (from this PR) the change that added "utf8: true"?

Yes, I think it would be better to remove it from this PR as it's out its scope.
I'm gonna work on removing the support for Symfony 3 (and revert the CI changes I've made) later this week.

Thanks!