The message shows up if you run in the background a program that keeps a peek into the clipboard, as for example the "Skype web plugin" and other telephone programs. A possible cause can also be the software "Audials".

You can find a thread with lots of complaints and a few flimsy justifications and non-answers here:
https://answers.microsoft.com/en-us/msoffice/forum/msoffice_excel-mso_win10/clipboard-problem-in-excel-2016/224b2a59-8a0c-4897-baba-a481dd06e085

The message shows up with Ctrl+C, Ctrl+X, using the Format Painter and only taking whole cells. It does not show up if you copy a part of a cell content.

Contrary to some Microsoft officials’ posts, the error message shows up multiple times per Excel session. Usually it shows up at the second Ctrl+C after opening an Excel instance, sometimes on the third Ctrl+C and rarely on the first Ctrl+C. And after that randomly every few usages of Ctrl+C. That can be several hundred times a day!
Not everyone is lucky as I am to identify the cause (telephone) and be also able to switch off its feature "clipboard surveillance".

There is no reasonable reason for this message at all.
1. The message is incorrect in 100% of the cases. The copying operations in Excel are not affected by the access of any third-party program to the clipboard. After clicking the message away, all insertion processes in 100% of the cases worked absolutely as expected and error-free. I've tested this with many different contents (formulas, validations, formatting, connected cells, etc.), from and into Excel.
2. The message is pointless in 100% of the cases, because it shows neither causes nor solutions. Just as helpful as a message "There's a light on in the basement."
3. If there was any case where the access of a third-party program to the cache was actually an impairment to the copying process in Excel, one would notice this anyway by missing the insertion. But the message would be as well unnecessary because it only would have reported the obvious and does not reveal any causes or solutions.
4. If the access of any third-party program on the clipboard would actually be a problem, then why don't you also ‘protect’ Word, Outlook, PowerPoint, etc.?
This problem is discussed and complained in forums for about 4 years (since Excel 2013). Why don’t you comment out that one line of code, that calls the message? Or add a code bracket, issuing automatically “User has clicked OK”. No big work, I assume.

When using Paste or Insert Copied Cells, conditional formatting rules should be merged by default. The paste behavior is currently inconsistent. For example, if you copy/paste a single cell, then rules are merged, but if you copy/paste multiple cells, then rules can become fractured or split (meaning that duplicate rules are created for the newly pasted cells).

There is already a precedent set for using merged conditional formatting as the default paste behavior: Google Sheets has been working this way for a long time and it works well.

Fractured or Split CF rules can be a serious problem. Consider the case where you are using one of the built-in data bar options to visually analyze data (see the attached screenshot). The sizes of the data bars are relative to the range of values in the Applies To field. If you insert a copied row or use other methods of copy/paste that cause the original CF rule to become fractured or split, then the relative sizes of the data bars will be messed up and lead to incorrect visual interpretations.

The "Paste and Merge Conditional Formatting" (or the equivalent option via Paste Special) is not something most users will know about. It also only merges rules when pasting within or overlapping a range (not when pasting contiguously). The rules should be merged when pasting rows to the end of a table (an example of pasting a contiguous range), or when using Insert Copied Cells to append rows to a table.

GS merges rules even if the ranges are not contiguous, meaning that if you copy A1 to C1, then the CF rule would apply to "$A$1,$C$1", and I think that should be the default behavior because it more closely models user intent (consider the data bar or color scale example).

If merging non-contiguous ranges is a problem due to a limit on the number of separate ranges that can be listed in the "Applies To" field for a rule, then perhaps the automatic merging would apply only to contiguous ranges (cells pasted within, overlapping, or adjacent to the original rules).

Although comments serve as "pseudo footnotes" today in Excel, it is very cumbersome when there are many of them, and you have to enable the "Print Row and Column Headings" setting in order to know which comment belongs to which cell. Enabling this setting also makes the printout look too busy.

Instead, it would be extremely helpful if Excel would include the superscripted footnote number (1, 2, 3, etc.) right in the cell (regardless of the cell's Number format), and allow the footnotes to then be printed at the bottom of the page on which they occur, or all at the end of the Excel printed worksheet (similar to what MS-Word does today).

In essence, this suggested footnote feature would replace Excel's existing Comment feature.

Need ability to allow only paste values in excel without using vba i.e. in an .xlsx. This makes protected worksheet templates better.

We send protected templates to our suppliers who fill in invoice data and send back to us. The Excel template cannot have VBA code or macros to ensure that it passes through email systems. When we get back the template we use an inhouse VSTO addin to validate / push the data into our Accounting System. The data validations / conditional formatting built into the template get overwritten if the supplier does a normal paste. However if we can enforce only paste values without using macros / vba code that will make the template more robust. Hope the new Excel has this ability to accept only paste values in a protected workbook / worksheet. Thanks