Sorry, I didn’t make myself clear. I mean that the situation now is that follow-up e-mails can be requested on comments to posts. But I use the comments section on my pages (the front page and other pages). That works well, but the follow-up check button doesn’t appear on those pages. I traced that behaviour to the file and line mentioned in my original request. The function comment_subscribe_init therein is restricting the e-mail follow-up on comments to posts only (so pages are prohibited from using this mechanism): 'post' == get_post_type().
I didn’t mean that I want to give users of my website the opportunity to subscribe to follow-up comments site-wide, but to follow-up comments on pages (not only posts).
So I made the change to wp-content/plugins/jetpack/modules/subscriptions.php as mentioned above. I tested the change, and it works well. Now the users of my website receive follow-up comments on my pages (and my posts) by e-mail.
