As a slack guest user I can not update the channels topic, what if I create a bot and use https://api.slack.com/methods/channels.setTopic ? will it be possible to update the topic or will it take my guest user for this and return error?
The rights of a Slack app are defined by
the requested scopes at installation (e.g. channels.write
, which you need to change the channel topic).
the rights of the installing user which are inherited (e.g. admin rights to delete messages)
So if you have a conversational bot that can be instructed to change the channel topic a guest user could use it and it would work.
Note that this will not work with slash commands though, since guest users can not use slash commands. (as described here)