Seems like it would make more sense for this to be handled in the slack input channel. It is a similar problem from here: Entity extraction problem due to Slack adding link
I.e. in the slack input channel, it should look for a regex of e.g…
<http://{something}|{something}>
and turn it back to {something} before it gets to Rasa. There’s already a similar one in the input channel that strips the user tags from the input. Would you be interested in working on this problem and PRing a fix?