Unless @steemchiller has something in SDS, I'm not aware of any way to get that information from the past. The author of that post used steemsql, but unfortunately steemsql went away during the Witness Wars.

At this point, the only place I know to get historical supply amounts is here, but that only covers about 2 weeks. So, I could calculate the 2-week interest rate from that fairly easily, but anything longer will take some time.

This isn't the first time I've missed steemsql.

Edited to add:

Based on virtual supply from the link above, the APR over the last two weeks has been (very roughly) about 7%.

$ perl -e 'print 26 * 100 * ( 466902074.12800002098 - 465653009.56300002337 ) / 465653009.56300002337 . "\n";'
6.97422287047544

It's roughly the same based on current supply, too....

perl -e 'print 26 * 100 * ( 420211866.27100002766 - 419087707.93999999762 ) / 419087707.93999999762 . "\n";'
6.9742242619498