Needs option to only share new posts

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.

Hehe, yes. I mean, we could set you up with a code snippet that would disable sharing for all posts before, say, Feb 1, 2023. Such a thing is possible.

The “easy way out,” however, is to just ensure the checkbox is disabled (either by doing so explicitly or by copy-and-pasting the following into a site-specific plugin or your (child) theme’s functions.php:

add_filter( 'share_on_mastodon_optin', '__return_true' );

In the next version of the plugin (later this week?), this (“unchecked by default”) will be a “regular” setting, no need for custom code.