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Here's info on noopener, I only noticed this coming up very recently on a lighthouse report.  As I understand it, it tells the browser to use a separate process for whatever the linked page has on it, rather than the existing one which could hurt performance on your own page if that one is very heavy.

https://developers.google.com/web/tools/lighthouse/audits/noopener

Seems like SEO is about to get that much harder:

http://analytics.blogspot.com/2011/10/making-search-more-secure-accessing.html

Any thoughts on this?

Although I think it depends a little on how 'important' it thinks your site is, I think google crawls sites with regularly updated content very frequently, so I'd suspect that as long as you're linking to this content from somewhere prominent on your site, you shouldn't have an issue.

You could also use a sitemap to tell google about fresh content, perhaps also consider applying to google news if you can, and then use a news sitemap which google will certainly be very quick to check and spider new urls from. In the advice for google news, they also suggest posting content to google+, so I'd also use that to post new content to, as it's another method of telling google you have something fresh for them to spider.

Placing a robots.txt in the root of the subdomain will do it:

User-agent: *
Disallow: /