A member may start a discussion thread about more than one registrar.
However, if a member starts a thread about a specific registrar and their experience with them, but does not request alternative registrars, it would be considered a rule violation if members were reported for taking the register specific thread off-topic by talking about a completely different registrar, especially if all the posts after that one took the thread even further off-topic and made it about a different register than the one the thread starter wanted to talk about.
The rules for that would be: 1.14. Do not make noncontributory posts/threads or artificially inflate your post count ("post pad"):
Do not unnecessarily post multiple times in a single discussion in a short period of time, sometimes called instant messaging (instead, take your time before posting so that all relevant content can be grouped together within a minimum amount of posts). Exceptions are made where reasonable (e.g., edit time limits have passed or each reply is dedicated to a different member's post).
Only post comments and questions that are relevant to the topic and contribute to the discussion (e.g., posting a single word like "bump" is not contributory; instead, post a short question or unique comment to encourage new activity in a thread).
1.15. Organization. (a) All content must be organized in an appropriate section (“forum”). (b) Posts and messages must be on topic and relevant where submitted (e.g., a discussion thread or direct message).
Note: The topic of a page is determined primarily by its title (and sometimes secondarily or refined by its first post/message); subsequent posts (even by the topic's creator) do not change the topic. If a topic's creator changes the title or first post/message within the edit time limits, then previous posts/messages will be unaffected and the new topic will apply moving forward only.
Was there a specific example you had, that you could share, that's different than what we outlined above?