Is there any service enabling a "Reading mode" for Chrome across devices?
The possibily best solution would be a bookmarklet. Due to same-origin constraints, it cannot load external scripts, which is why the forked Readability bookmarklet does no longer work. Neither does Readable, which was suggested in an answer.
On mobile, not all webpages are viewed in the browser directly. In these cases it would be useful for the solution to be accessible through the system-wide "Share" menu.
Browser extensions would be possible on Desktop, but come with major caveats.
- Privacy. Browser extensions have been known to get sold to or hacked by malicious companies, and thus to turn into spyware without warning, to the point that bloggers recommend against using any, if possible. A reader-mode extension needs full access to all websites, hence they are among the extensions most vulnerable to just abuse.
- No mobile support. By their nature, extensions don't help on mobile devices, where Chrome doesn't allow any.
- Invisible distractions. Some extensions (Just Read, Purify) have been suggested. Both provide a more readable overlay over the original article, but don't replace it. As a consequence, some intrusive practices are not prevented by them -- such as CPU heavy ads, which for fairness sake I do not want to block wholesale with an adblocker.
Reference solutions, and why they don't apply.
On iOS, Safari provides a reading mode, that strips articles to the bare basic for distraction-free reading. As a bonus, it also works in apps like feedly that open an in-app Safari overlay.
Chrome on Android now implements a reading mode. Like with safari, it is offered for Chrome Custom Tabs too. Which would be great, if its heuristic agreed with me on which webpages need it. As it is, it is useful when it is offered, but it isn't offered half the time, and even inconsistent within articles of the same webpage.
Chrome on iOS devices and Windows PCs has no reading mode at all out of the box.
Why ad-blockers don't cut it.
Leaving aside the ethical conundrum of depriving websites of their income, ad-blockers don't affect non-ad distracting design choices, such as
Side-bars. Often with navigation elements and links to other articles of the website.
Overly wide lines. If columns are too wide, more frequent loss of the vertical position adds distraction.
Small font size. (especially on mobile); Zooming in results in horizontal scrolling, which is about as badly distracting as it can get. Some webpages even disable zooming in by using a meta-tag.
Floaters. Elements with a fixed on-screen position, that often significantly reduce the effective screen size and force distracting clutter into the field of view. Typically used as navigation bars (a less intrusive example being used here on stackexchange too), for social media "share" widgets (especially useless on mobile, where we have a share menu anyway), and some of the most intrusive ads (adblockers may end up blocking the content, but not the floater itself). These are typically implemented with the CSS setting
Existing extension-based solutions
Some Chrome extensions (Just Read, Purify) have been suggested.