Linking to an HTTP site (as opposed to HTTPS) won’t damage your own domain’s organic search performance
According to John Mueller, there is no negative SEO impact to be had from linking to an external HTTP site (as opposed to HTTPS). If the webpage you’re linking to is on HTTP, there won’t be any negative repercussions for your own domain.
Inconsistent HTTPS Migrations Cause Bigger Ranking Fluctuations
Google is more cautious with inconsistent HTTPS migrations that don’t map one to one from HTTP to HTTPS with clear 301 redirects. HTTPS migrations that also remove a lot of URLs or block URLs by robots.txt are likely to see bigger fluctuations in rankings.
GSC Messages For HTTP Version After HTTPS Migration Could Mean Signals Pointing to HTTP
John recommends having the HTTP and HTTPS versions of your site verified in Search Console. If you still receive messages after a HTTPS migration it could mean that some signals are still pointing to the HTTP version.
HTTPS is Used as a Tiebreaker Signal in Deciding Rankings
Switching to HTTPS won’t boost rankings significantly but it is used as a tiebreaker when Google is trying to decide which page to rank if all other factors are at a similar level.
Google TLS Warnings Won’t Impact Rankings
TLS warnings are being sent in GSC to alert webmasters about common issues including HTTPS or certificate configuration problems. Receiving this warning doesn’t impact rankings.
Google Uses Multiple Signals to Choose HTTP or HTTPS URLs
Redirects, internal links, sitemaps, rel canonicals are taken into consideration when Google chooses to index a page on HTTP or HTTPS. Internal links to HTTP URLs after a migtration to HTTPS will give Google conflicting signals, but usually there will be enough signals to indicate that the HTTPS version should be indexed.
Most HTTPS Migrations Take a Day to Change in Index
A HTTPS migration is easier for Google to process than most other types of migrations because it keeps the same domain and same URLs. If a site is restructured with changes to internal linking or the domain name, it means Google has to think about a lot more. However, HTTPS is still a big change and takes time to be processed by Google – most take a day or so to switch over in Google’s index.
If Site Doesn’t Have Valid SSL Certificate Google Will Stick to HTTP Version
If Google discovers an HTTPS URL but it doesn’t have a valid certificate then they’ll probably stick to the HTTP version.
Google Sees SSL Certificates as the Same as Long as Have Valid Certificate
Google sees all SSL certificates as the same as long as they are valid and accepted by modern browsers.