best response confirmed by Warwick Ward (Regular Contributor)

Hi @Warwick Ward, I recommend taking a look at our What's deprecated or removed from SharePoint Server Subscription Edition article if you haven't already. It lists the features that have either been removed or deprecated (i.e. could potentially be removed in the future). There have been no other significant feature removals/deprecations in SPSE, including to our classic UI, site templates, etc., so we have no plans to end support for classic UI at this time. Just keep in mind that future investments will focus on the modern UI instead of the classic UI, so we recommend using the modern UI for the best experience for usability, accessibility, etc.