Is it because they will no longer support the .NET Framework (based on 4.6.1 or 4.8)?
Is .NET 5/6 better?
Activity Classic → Modern
3. Is there a difference between classic and modern activities?
4. Will classic activities be discontinued?
5. Should we think that modern activities have better functionality than classic activities?
A formal date is currently not announced, but we can assume a phase out in the future
The word better should be bound on factors. So in short:
has advantages and disadvantages. But as we can expect the progress on the functionality we would focus Modern Design for UI-Interaction / Application related activities. For the latter mention we can also work within an Hybrid approach (e.g. Excel Classic and Excel Modern)