Microsoft announce (June 2024) that the next release of Skype for Business Server will be a Subscription Edition (SE) which will be available early in the third quarter of 2025. From a licensing perspective it will follow the same rules as for SharePoint Server SE: you’ll need active SA to use the product – or buy the subscription licences – and if the SA lapses, then you retain rights to the 2019 version of the product.

Find the announcement article with migration path recommendations here: https://bit.ly/3WMVrvf.