Is the following sentence acceptable in semi-formal or formal written speech?
How to adopt a lifestyle that consumes less?
Or ought it be rephrased? For example, to the following?
How do I adopt a lifestyle that consumes less?
The context of this sentence is a headline or title, which follow different rules to normal English, and it works perfectly well for that.
You shouldn't use that sentence in regular English, formally or informally; the rephrasing you suggest would be a perfectly acceptable alternative.
Here's an interesting dissertation on the grammar of headlines showing some of the differences.
I would say that
could be used as a title or heading in a written article or question post. This phrase is only a fragment, because it does not have an independent clause containing a subject-verb relationship. Because it is a fragment, it should be punctuated as a statement (if at all). If someone is actually asking the question, then use
or similar, which is phrased as a question and contains a subject and appropriately conjugated verb.