P: Toolbar [Done] Button has moved to tool bar beneath Histogram

Adobe has done away with the "Done" button in most of Lightroom in the most recent updates.  Instead, the Editing Panel has its own button now.  You just click on that button to get back to the editing panel.  It does take some getting used to, but I've found I like the update pretty well.

Cheers!

What "OK" button in what function? Where did you see it before the update?

Are you perhaps talking about the "Done" button? If so, that has been removed in many places in Lightroom Classic.

Version 12 of what? I'll move your post if tell me where it's supposed to go.

The [Done] button has been removed in v12- It is redundant! (FOUR other ways to [Done].!!}

1) Click the (new) Edit Button instead, to return to Developing-

2) Or- Click on the Crop Tool icon once again,

3) Or press the [Enter/Return] key.

4) Or Press keyboard key [R]  which both opens and closes the Crop tool.

To enable changing the "Ratio"- click on the Padlock to 'open' it allowing a 'Freehand' crop.

Click on the context menu next to the padlock for crop ratio presets.

Use Custom ratios, that you define, and LrC will store up to Five of your custom ratios.

This has been discussed many times already. Either the Edit icon (left-most in the local adjustment panel) or the Masking icon (right-most in the local adjustment panel) or "Close" produces the same effect as the "Done" button used to produce. The "Done" button has been removed.

AFAICT a number of people seem to have concluded that clicking [Done] somehow related to "applying the change" or some such notion - IMO not only mistaken but misleading. 

In typical applications, modally available controls - such as a PS filter or a dialog box or whatever - impose a decision point before you can return to the normal workspace. Only if you clicked Apply, Done, OK or some such - would the change(s) be committed. That would not happen if you were to pull the plug first, or if you clicked a Cancel button. 

LrC works very differently in this respect. The instant you've released the cursor button from an individual action such as choosing a different crop aspect, adjusting a linear gradient's angle, altering a slider value, whatever - that's already committed and a fresh History step is already showing accordingly. The deed is already done. 

Whether you thereafter want to keep using that same tool or to exit it, is a quite separate matter from this "deed". All that the now-missing button ever did, was to stop offering a particular tool's interface. IMO the word "Done" was always ill chosen so far as what it implied. The "Close" button makes logical sense within the relevant tool panel itself, because that is exactly what it does. But duplicating this onto the Toolbar and calling that button "Close", might have been taken as closing the toolbar itself rather than closing the relevant tool panel.

When it's so unclear what a given button should even rightly say... how can its action be anything but ambiguous?

(if you look at what other Toolbar controls remain: they all have to do with interface - not, applying edits)

SeaTurtleBud, stated "This seems like a bad design decision. Now users must hit the keyboard to execute (DONE) instead continuing to edit with just the mouse?!?  Ouch! "

Absolutely not if you are using an editing tool, e.g. the crop tool you click on the crop icon to activate carry out your crop task when you have completed your work and click on the crop icon it will save your work and return you to where you were working previously. If you click on another tool then the crop would save and you will be sent to the new selected option. No keyboard action is involved.

In my workflow I rearly use keyboard keys, no need for the "Done button".