If you want to tweak your robots.txt for protection against unwanted search engine indexing, the widely-circulated answer is that only Marketo Support can do this.
That's not actually true, though. You can do it yourself.
Simply upload a custom robots.txt to Design Studio:
Then create a Redirect Rule (you'll need a rule for each of your LP domain aliases):
And yes, 301ing your robots.txt is okay by Google: