There is. This will not happen if you uncheck the “Share on Mastodon” checkbox before updating.
Granted, the checkbox is ticked by default. It can, however, be disabled, so that you would have to check it explicitly (also for new posts) in order to enable crossposting; if you’re familiar with adding hook callbacks to modify WP’s behavior, there’s the https://jan.boddez.net/wordpress/share-on-mastodon#share_on_mastodon_optin filter.
In the upcoming version, this will be a “proper” UI setting.
If you wanted to always disable syndication for posts before a certain “cut-off date,” there’s https://jan.boddez.net/wordpress/share-on-mastodon#share_on_mastodon_enabled, which, together with a bit of PHP (a check against $post->post_date, namely) could be used to do just that. You’d have to determine this cut-off date yourself; the plugin does not keep track of when it was installed.
Ah, yes, on the post itself, is a checkbox. I was looking in settings. Not exactly what I meant, but now that I know how it works, I’ll just have to be careful.