Confirmed.

64-bit, KDE Plasma 5
Build ID: 5.1.2.2 Arch Linux build-1
CPU Threads: 8; OS Version: Linux 4.5; UI Render: default; 
Locale: fi-FI (fi_FI.UTF-8)

Confirmed "me too"
Window 10 x64, 5.2.2.2
If you use the "Basic shapes" version you can only drag, but not enter the value.
You have to use View -> Toolbars -> Legacy rectangles to get one where you can enter the exact value, which is quite often needed.

With my Libo 5, i just updated old documents, and tried to create new rounded rectangle... And i discovered that the radius field is disabled.
I loosed time to understand the issue.

In fact, olds documents contains "draw:rect" in content.xml, and new "rectangle" (round or not) are create with "draw:custom-shape".
The manual adjustment of the rounding is not possible for my need.
my soluce : copy and paste my old shapes, what a mess

(tested with libo 5.3 - windows 10)

still repro in 6.1.1.2

still repro in 6.3.3.2 (x64 W10)
A quick manual basic rounded rectangle create a soup of content like that :

<draw:custom-shape draw:name="Forme1" draw:style-name="gr1" svg:y="1.032cm" svg:x="1.011cm" svg:height="2.834cm" svg:width="6.97cm" draw:z-index="0" text:anchor-type="paragraph">
  <text:p/>
  <draw:enhanced-geometry draw:enhanced-path="M ?f7 0 X 0 ?f8 L 0 ?f9 Y ?f7 21600 L ?f10 21600 X 21600 ?f9 L 21600 ?f8 Y ?f10 0 Z N" draw:modifiers="3600" draw:type="round-rectangle" draw:text-areas="?f3 ?f4 ?f5 ?f6" draw:path-stretchpoint-y="10800" draw:path-stretchpoint-x="10800" svg:viewBox="0 0 21600 21600">
  <draw:equation draw:name="f0" draw:formula="45"/>
  <draw:equation draw:name="f1" draw:formula="$0 *sin(?f0 *(pi/180))"/>
  <draw:equation draw:name="f2" draw:formula="?f1 *3163/7636"/>
  <draw:equation draw:name="f3" draw:formula="left+?f2 "/>
  <draw:equation draw:name="f4" draw:formula="top+?f2 "/>
  <draw:equation draw:name="f5" draw:formula="right-?f2 "/>
  <draw:equation draw:name="f6" draw:formula="bottom-?f2 "/>
  <draw:equation draw:name="f7" draw:formula="left+$0 "/>
  <draw:equation draw:name="f8" draw:formula="top+$0 "/>
  <draw:equation draw:name="f9" draw:formula="bottom-$0 "/>
  <draw:equation draw:name="f10" draw:formula="right-$0 "/>
  <draw:handle draw:handle-range-x-maximum="10800" draw:handle-range-x-minimum="0" draw:handle-switched="true" draw:handle-position="$0 top"/>
  </draw:enhanced-geometry>
</draw:custom-shape>

Give back simple "<draw:rect/>"

The same applies to all components of LibO, so I changed the component accordingly.
Still present in 
Version: 6.3.4.2 (x64)
Build-ID: 60da17e045e08f1793c57c00ba83cdfce946d0aa
CPU-Threads: 4; BS: Windows 10.0; UI-Render: Standard; VCL: win; 
Gebietsschema: de-DE (de_DE); UI-Sprache: de-DE
Calc: threaded

And corner radius can still be set via context menu -> Position and Size

(In reply to Gerhard Weydt from comment #9)
> And corner radius can still be set via context menu -> Position and Size

Sorry, I was wrong.

(In reply to chbok from comment #7)
> Give back simple "<draw:rect/>"

It still exists. Draw:Impress: It is in the toolbar "Legacy Rectangles". You can open it via View > Toolbars or add it as sub-list to a toolbar via Customize. Writer/Calc: The rectangle is available as single command "Rectangle" via Customize.

The "Slant and Corner Radius"-tab of the Position&Size dialog has got two sections "Control Point" in the meantime. They allow to determine the position of handles of shapes, which are "draw:custom-shape". So for to enter the corner radius for such an rectangle, you have to use the field "Control Point 1, X:" and not the field "Corner Radius".

The field cannot be named "Corner Radius", because it works for handles of other shapes too.

With getting these new fields in the dialog, the bug is a "works for me" now.

Ho ! you explain me how to add an "legacy rectangle", but why still subsits two manners for drawing them ?
A casual user under Writer never will understand that, if he must open Draw:Impress to draw a shape, and next copy/paste it in Writer.
okay, i tested your procedure and it works. But the bug refer to Writer. So, i claim for return the "legacy forms" back in Writer.

(In reply to chbok from comment #12)
> Ho ! you explain me how to add an "legacy rectangle", but why still subsits
> two manners for drawing them ?
> A casual user under Writer never will understand that, if he must open
> Draw:Impress to draw a shape, and next copy/paste it in Writer.
> okay, i tested your procedure and it works. But the bug refer to Writer. So,
> i claim for return the "legacy forms" back in Writer.

There's no need to add a legacy rectangle. Simply add a rectangle via menu or tool bar, it will be of the new type "custom shape". And if you won't/can't use the mouse, then change the corner radius - as pointed out by Regina - by changing the control point: the value shown in the X or Y coordinate (it depends on which side is longer) is the distance between the control point and the corner point, which is also the radius of the circle used to draw the rounded corner.

As explained in comment #11, the added Control points indeed fixed the bug I've reported. Thanks for closing this!

Please, think to a basic user. When drawing a basic shape like a rounded rectangle, he only see a disabled "corner radius" field and must guess that to adjust precisely this radius, he must :
- use Draw:Impress to copy a "legacy rounded one" to have access to the "corner radius" field
- or enter the radius value in a mystical "Control Point" field (mystical for this particular user)

I just tested this with the latest release (7.0.3.1) and the corner radius is still broken in Libre Office. Why is the "Legacy Rectangle" NOT the default option to draw a rect?

I have hundreds of figures for my lectures done in Star Office and Open Office. If I want to use Libre Office for that purpose I would need to manually adjust the toolbars to include Legacy Rectangle options just to be able to add or remove a corner radius in some years.