We deleted all negative comments in our self-moderated thread because of a recent user made a lot of trouble.
You're free to choose the features you want when posting a new thread. However, since you represent a well-established brand (as you said), you should be aware that posting a self-moderated thread as an official ANN channel is unethical and raises many red flags that might discourage users from joining the site.
If there are posts you consider spam that could taint your ANN, there's a "report to mods" feature you can use to report it to the moderators, and I'm confident they'll make the right decision.
The negative comments are still there is someone wants to do the work. Everything is archived and can be found, just a few clicks.
Deleting posts is allowed but it does look like they're hiding stuff if they are deleting all the negative. I dont know if it discourages people from joining the site because unsuspecting individuals may have no clue what's going on.
Deleting comments you don't like only has the opposite effect. This usually ends with the opening of numerous new discussions, which then always have a negative connotation towards the service. So the attempt to hide problems and unwanted criticism turns into a negative campaign.
I have not yet seen that someone managed to hide such a thing with the help of self-moderation
I find that some brand representatives treat forum features with indifference to the consequences of naive usage. Often, they are team members tasked with managing official channels without any prior experience, particularly not being a forum user. Maybe for this reason, it is always advisable to appoint a veteran member (community manager) to post official announcement and manage necessary updates, while the role of the representative remains to respond to community questions (disputes,feedbacks,complaints...) and engage with diverse opinions.
So I guess my suggestion is the right option for any brand channel to report unwanted comments to mods who know better how to classify the posts qualities.