DEPR: let's deprecate
xref #18202 

We have some cruft, let's deprecate it (I have noted some which already have an issue associated).

### From ndarray

- [x] `Series.compress()` (#21930)
- [x] `Series.imag` / `Series.real` (#27106) 
- [x] `Series.nonzero()` (#24048) 
- [x] `Series.put()` (#27106)
- [x] `Series.itemsize`
- [x] `Series.flags`
- [x] `Series.strides`

### timeseries specific

- [ ] `Series.first()`
    let head / tail take a timedelta
- [ ] `Series.last()`

### other

- [ ] `Series.swapaxes()`
  - No reason once panel is gone.


### non-controversial
- [x] ``MultiIndex.to_hierarchical`` (only used for ``Panel``) (#21613)
- [x] ``Series/DataFrame.compound()`` (#26405)
- [x] ``Series.ptp()`` (#21614)
- [x] ``Series.from_array`` (#18213)
- [x] ``Series.valid()`` (#18800)
- [x] ``Series/DataFrame.slice_shift()``  (#37601)
- [x] ``Series/DataFrame.tshift()`` (https://github.com/pandas-dev/pandas/pull/34545)
- [x] ``Series/DataFrame.get_values()`` (https://github.com/pandas-dev/pandas/issues/19617)
- [x] ``Index.dtype_str`` (#27106)
- [x] ``Index.summary()`` (#18217)
- [x] ``.get_ftype_counts`` (#18243) (#20404)
- [x] ``.get_dtype_counts`` #27145 
- [x] ``Index/Series.asobject`` (#18237) (#18572)
- [x] ``Index.to_native_types()`` (make private) (https://github.com/pandas-dev/pandas/pull/36418)
- [x] ``DataFrame/Series.as_matrix`` (#18458)
- [x] ``.clip_upper/.clip_lower`` (replace by ``.clip``) (#24203)


### Potentially
- [x] ``.ftypes`` (#18243) (#26744) 
- [ ] ``.xs()`` (#6249)
- [ ] ``.iat/.at``
- [ ] ``.take``
- [x] ``.lookup`` (#35224)
- [x] ``DataFrame.from_items``
- [ ] ~``Series/DataFrame.add_prefix/add_suffix``~ (https://github.com/pandas-dev/pandas/pull/18347)
  - Maybe add suffix / prefix to concat?
- [ ] ``NDFrame.filter``