I've noticed that verifyReceipt has been marked as deprecated. Could anyone provide information on the specific date when this function will no longer be available? Additionally, if the exact date is still undecided, will developers be notified in advance, such as 6 months or 3 months prior to its discontinuation?
Nothing has been announced; we don't know.
You could look at previous deprecations to see if there is a pattern. Looking only at online services, i.e. not on-device SDK APIs, Apple have generally announced the removal dates straight away and they have been quit short periods (examples: IAP hosted content, XML App Store API). So it's actually unusual for them to announce a deprecation without a removal date.
Are you calling verifyReceipt directly from your app, or from your server?