How did their 401k match work?

They used to wait until end of year to match funds until recently they switched to monthly matching. So if you left before end of year, your match didn’t pay out iirc. Now they’re getting rid of 401k matching entirely so suppose it’s a moot point.

Didn't they get to write off their contribs to the 401k? And if they're now contributing to the RBA instead would they still get to write that off?

I think outside of a few edge cases employee compensation is always a writeoff for the employer

You had to be there on 12/15 of each year to get it, else zip.

last year they started monthly matching, of course they ruin everything a year later

Almost certainly they reverted to monthly to clear any hypothetical carried balances that could interfere with their new scheme.