We've had a few people have trouble with certain versions of the MacOS Gatekeeper program blocking the game. Sorry about that.
Launching the game through the Itch App seems to get around that issue.
Alternatively, you can email riotsupport@means.media, preferably with the email address tied to your Itch account, and we can send you Steam or GOG keys (or Switch if you have one), and that also gets around the issue.
Lastly, for at least one person, just setting the Gatekeeper to "allow apps from anywhere" worked. But for another person it didn't work, and that could be a security vulnerability, so we recommend those first two options.