When LibreOffice Calc opens an Excel 97-2003, it would be beneficial if the maximum number of rows and columns are 65536 and IV respectively.
This would assist files to be saved without any data loss.
When an Excel 97-2003 file is opened, the window's title could show something like:
Untitled 1 [Excel 97-2003 Compatibility Mode] - LibreOffice Calc
If the file is saved in ODF format, then the maximum number of rows and columns would then be increased.
Discussed with devs on IRC and they suggested that this should fall under the category of improving warning messages in general.
*** This bug has been marked as a duplicate of bug 44817 ***