ENH: Undeprecate and reimplement DataFrame.lookup

[impredicative]

Is your feature request related to a problem?

I seriously get the impression that Pandas is actively being sabotaged. As a case in point, DataFrame.lookup was deprecated in v1.2.0. The problems with this are:

1. The doc for DataFrame.lookup says to see itself for an example. Well, there is no example in the doc page. The only example I'm aware of is here instead which is a different page.
2. The changelog for this deprecation points to GH18682 which looks to be wholly irrelevant to this change. A lot has gone wrong here.
3. There is significant user code that already uses lookup. Why break it? If the current implementation of lookup is suboptimal, shouldn't it be optimized instead?
4. It is extremely complicated to use melt when lookup works quite simply. For example, compare this simple answer using lookup with this complicated answer using melt.

Does nobody review changes, docs, and release notes anymore prior to release? It looks this way.

Describe the solution you'd like

1. Undeprecate lookup.
2. Optimize lookup if attainable using melt.

[impredicative]

Accidental duplicate of #39171