they're not optional.
https://www.cbsnews.com/news/find-commands-easily-in-word-20...
I can easily imagine a world where other office apps are shutdown, you have the golden children like Excel, Word, Outlook and Powerpoint. Which change UI and design language or change features over the years.
Then there's things like Office Whiteboard, VIVA, Bookings, Lists, Planner, Power Automate, Yammer, Sway. Who knows how long they'll last?- even mainstays such as Microsoft Access have been deprecated, and I bet you didn't know that you had half of those tools.
I mean, I learned graphics design on Macromedia Fireworks. Can I use that now? no. It was the industry standard for raster graphics that were not digital photos- but it's dead now.
Even staying with Excel: Excel on Windows and Excel on MacOS are similar, but not the same. Many features available on the Windows version are simply unavailable on MacOS.
Where-as something like Libreoffice Calc works consistently across platforms.
So, yeah, learning Excel only means that you're Microsoft Windows and Microsoft Office for your entire professional career.
Which is great for microsoft, because you will always take the pain of learning whatever they do over migrating to something else, due to the cost of retraining to more conventional standards.