I am also able to repro this on our own hosting, on a business plan trial. @jancernik look for an invite in your email so you can repro it there too.
On our hosting, you have to also enable the email in admin setting in order for the Custom incoming email address group setting to appear. That threw me for a loop.
Also the copy for this setting is confusing on our hosting because no additional manual or pop3 polling needs to be set up - it works out of the box. I will update it.
Edit: I updated the copy in this PR to simplify it:
Allow users to post new topics via email. After enabling this setting, you will be able to configure incoming email addresses for groups and categories.