Google's Gary Illyes said that you should not ignore the HTTPS ranking signal. He said on Twitter "the signal affects enough queries measurably that I wouldn't ignore it."
Here is the tweet from Gary Illyes:
I can't say the exact number but the signal affects enough queries measurably that i wouldn't ignore it
— Gary "鯨理" Illyes (@methode) June 5, 2019
In addition to this tidbit, he said that even though invalid HTTPS security certificates still do not invalidate your ranking boost, having an invalid HTTPS cert can impact other factors.
Gary said "a broken cert will affect other things though. For reasons, i know for certain that the ranking signal itself would be unaffected: you have https in the result, you get the (tiebreaker) boost."
A broken cert will affect other things though. For reasons, i know for certain that the ranking signal itself would be unaffected: you have https in the result, you get the (tiebreaker) boost
— Gary "鯨理" Illyes (@methode) June 5, 2019
The question came up because John Mueller implied you need it to be valid:
So now SSL certs need to be valid for https ranking signal? Previously Google said they didn't check if the cert was valid, it just had to be https in the URL
— Barry Schwartz (@rustybrick) June 4, 2019
So there you have it - make sure you have HTTPS and make sure it is valid and working properly for ranking, not just for your user.
Forum discussion at Twitter.