For anyone looking for an rss feed, I found this on the steam community forum after finding out about isthereanydeal.com:

RSS may apply filters and options, so getting the RSS for this is the best we can do right now: https://isthereanydeal.com/#/filter:&price/0/0;/options:strict,all

It all depends on what can our tracker parse - and that depends on how will Steam show these free games on the store.

Here’s a RSS for EU2 region: https://isthereanydeal.com/rss/deals/eu2/?filter=price%2F0%2F0&options=strict,all

It seems to work well!