Can you post an image of the lighting symbol? I honestly have no idea where that is; I have never seen it before. Is this something that is a lighting symbol on all wikis or does it change depending on the wiki?
Never mind, I think I know what you are talking about. Do you mean the thing next to the add page button in the local navigation? It should still lead to Wiki Activity on wikis that have the page. However, UCP wikis don't have that page so it links to Recent Changes instead. Since UCP wikis lack the Wiki Activity feature, there is nothing to change it to even if you could.
There is no code to get Wiki Activity to work as it did on legacy wikis. However, FANDOM staff is still considering bringing it back in some form, they just won't say what or if they will actually do anything soon.