With regards to first party cookies, does Google analytics count as new users if the users not revisit on the website within 24 hours as a first party cookie is deleted within 24 hours?
If GA is looking for a cookie to track the user, and the cookie expires before the users visits your site (24 hours per your question), then we can expect GA to treat them as a new user.
However, it's only
_gid that expires after 24 hours;
_ga does not, it expires after two years. If a _ga cookie is not present, new Client ID for the new user (aka "New Users" in your GA reports).
iOS 14 tracking changes
As far as I know, in iOS 14 Safari blocks all third-party cookies. I don't know of anything about first party cookies. There is added transparency and privacy reporting in Safari, what is tracking you is more visible. So if a user sees your cookie in their report (and doesn't want it) they can delete it.
As long as you don't have an app (so you're just a website) you only need to worry about changes in the Safari browser.
So really it's just Intelligent Tracking Prevention (or ITP) which has been updated and Apple's new Privacy Reports.
There are not any workarounds to ITP related challenges at the moment. So for now, the best you can do is monitor your data and see if/how the iOS 14 updates are affecting you and then go from there.