e.g. some old versions of Crystal Reports can only do XLS. There must be loads old systems around that can only do XLS
Ugh - I still deal with clients who are creating brand new XLS and DOC files and expecting to have all the latest features (co-authoring when hosted in 365/SharePoint Online/OneDrive/Teams)...
At that time the was less information about 97-2003 file formats (OLE2-based), but it was fixed around 2008-2009: https://alexott.blogspot.com/search/label/file%20formats
Even the current version of Excel still inexplicably auto-converts long numerical strings to Scientific Notation.
I had someone come to me recently asking for some data "in an XLS". I asked them if they specifically need XLS or if they just need something that can be read by Excel. It was the latter, but they didn't know that there was a difference. To some people, XLS == Excel. Sort of like how TLS is still being referred to as SSL - somewhere in the world there is a developer being asked to use SSL for a new project because SSL == secure.
...
“Oh nice! Exactly what I need!”
<copy>, <paste>
...
“Woah it builds! Ship it!”