what would be the maximum time limit microsoft has suggested to change from classic UI to Modern UI?
closed as unclear what you're asking by MHeld, Mohamed El-Qassas MVP♦ Oct 26 '18 at 10:27
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Current advise from MS is that they will be supported, so for now you don't need to change. Enterprises have invested a lot in the classic experience site so MS needs to take that into account.
"We're not deprecating the "classic" experience; both "classic" and "modern" will coexist."
I think the problem will be that they will be supported but no new features will be developed for it as Modern and Teams get all the attention.
There are as well those threads which may give you more insight: https://techcommunity.microsoft.com/t5/SharePoint-Developer/Classic-mode-support-timeline/td-p/3640
There as well those discussions: