Can I downgrade from macOS Sequoia?

Well, it originally would have shipped with Sonoma 14.1 which you may be able to downgrade to, but no earlier. Why would you want to?

the old gaff wrote:

When in Excel 16.101.3, the keyboard does not work properly.eg when in a cell, if you want to write you have to hot shift+space bar to prevent typing not having a space. The keyboard is old : 2008 ???

Make sure that Full Keyboard Access is turned OFF in your Accessibility settings.

Possibly. What Mac do you have?

Whenever someone asks about downgrading we always assume it’s because of problems. Explain why you want to downgrade and perhaps there’s a solution that can be offered.

Does it work correctly with the MBP keyboard? I don't use Excel so I can't test anything.

If it works with the built-in keyboard then it's your 2008 keyboard that's a problem.

FWIW I’m running Excel 16.103 on Tahoe 26.1 with no such issue. Just tested and failed to replicate. Have you made sure Excel is update to date and current?

I'm not quite sure what you mean, but I'm using Excel 16.103.1 on Sequoia 15.7.2 and not having any issues entering data.