I read it here: https://semperplugins.com/wordpress-requires-sites-to-use-https.
That article is referencing a declaration Matt Mullenweg (co-founder of WordPress) made in December of 2016: https://wordpress.org/news/2016/12/moving-toward-ssl/
It is planned that certain features may be introduced to WordPress which require HTTPS. No movement has been made towards such things as of today (August 13, 2017), though the REST API will likely be the first to require HTTPS.
The only move made so far is that hosting providers recommended at https://wordpress.org/hosting/ must provide an SSL certificate and enable HTTPS by default for new accounts (which they do now).
Overall, WordPress will never force you to switch to HTTPS, but you may find that you won’t be able to use some features in the future.
With all of that said, switching to HTTPS is a very good idea. As of Firefox 51 https://blog.mozilla.org/blog/2017/01/24/gets-better-video-gaming-non-secure-web-warning/ and Chrome 56 https://security.googleblog.com/2016/09/moving-towards-more-secure-web.html the two most popular web browsers have switching from highlighting HTTPS sites as “Secure” to highlighting HTTP sites as “Not Secure.”
In other words, the internet is moving forward to making HTTPS the dominant protocol, and I recommend joining the party. So far, more than half of known sites now run on HTTPS: https://www.ssllabs.com/ssl-pulse/
To switch your WordPress site to HTTPS, you’ll need to first acquire a valid SSL certificate from your hosting provider or a certificate authority. Most hosting providers offer these for free or very low cost, and if yours doesn’t, it might be time to move to one that offers SSL certificates for free: https://wordpress.org/hosting/
Next, just follow this guide: https://make.wordpress.org/support/user-manual/web-publishing/https-for-wordpress/
I try to understand should I have to fix 1500 plus posts manually or that can be done in a more easy manner.
That can be done very easily with a plugin like https://wordpress.org/plugins/velvet-blues-update-urls/