Remove deprecated API

[bnayae]

I believe it's time to remove the deprecated API (like First, Last, FirstOrDefault, etc) from the main NuGet.
for backward comparability we can offer it in different NuGet(different assembly) for example

Nugat of System.Reactive.Linq.Deprecated
which will have both assemblies
System.Reactive.Linq.dll
System.Reactive.Linq.Deprecated.dll

[RxDave]

Please consider that First and FirstOrDefault do have their uses. For example, devs have used First before BehaviorSubject had its Value property added to extract the latest value imperatively; e.g., commonly for exposing it as a property on a wrapping type.

Perhaps if we remove these operators we should do something similar for AsyncSubject and ReplaySubject; e.g., HasValue and Values, respectively. For ReplaySubject specifically, reading the current buffer is now a matter of doing something like rs.Take(size).ToList().First(), which doesn't work if size is unbounded, and it's inefficient. Are there any other common uses of these operators that are warranted?

I'm not sure about Last and the other blocking operators, but it may be worth considering each one individually before removing them from the library for good.

[akarnokd]

This probably can't happen until 5.x.

[bartdesmet]

I'll revisit the list of members that have been marked as obsolete and decide on an action:

• Take some out or keep some (all, or a subset, TBD).
• Reduce code size for those that are kept (if any), e.g. by layering them on top of async variants directly rather than parallel implementations.