It's not a slur any more than calling someone a Nazi is a slur. In both cases, the fascists chose the term for themselves and use it to self-identify. Leftists likewise use the terms with all the derision they deserve. Simple as. If I call a Nazi a Nazi, it's not a slur just because I think Nazis are evil and disgusting and are in need of redacting. So too with Zionists.

If someone is using the term to describe themselves, but they (correctly) think that settler-colonialism is wrong and that the state of Israel is a genocidal ethnostate, then they are misusing the term, according to both the vast majority of Zionists as well as the people who oppose Zionism. As always, what is antisemitic is the equating of Zionism with Judaism, it is not antisemitic or saying a "slur" to accurately use the term Zionist as an epithet. It's disgusting but unsurprising how the Zionists keep harping on this to try to make themselves out to be the ones being persecuted.