I'm doing the latter (in large part so that I post old.reddit.com links), and I can assure you It Does Not Work As Expected Or Desired.
> I can assure you It Does Not Work As Expected Or Desired.
in what sense? Seems like it's been the same for ~8yrs, other than having to re-set the flag in settings now and again. Like I said, the only time i've seen an issue is the odd time someone posts an image gallery - I can't see it in oldreddit style unless I click on comments.
I both want myself and others to experience Old Reddit. So I use Old Reddit URLs, both for my own interactions and when posting links to the site.
And if I do that, I do not remain on Old Reddit.
With JS disabled for New Reddit (one mechanism for reminding me not to use it), the interface doesn't persist.
Note that this has similarly been a long-standing issue when using i.reddit.com, the lightweight mobile interface. So long as you're clicking on interface-related links, you're OK (e.g., posts within a subreddit, or short-style links to users or subs: /u/<username> or /r/<subname>). But if there's a hardcoded fully-qualified link to a given Reddit URL (post, comment, wiki, user, etc.), then whatever the governing host-part is is applied, and bang, I'm off my preferred interface.
Many subreddit sidebars explicitly code the fully-qualified hostname to their Wiki or various support pages. This is especially infuriating.
I don't use any blanket JS blocking browser addons. I find it frustrating how much of the web gets broken with them, and having to manually flag damn near every site just to get them to work is tedious. Just using uBlock Origin to block all the trackers and ads is good enough for me.
I never type "old.reddit.com". I'm always on www.reddit.com, and I'm always on the old site experience.
Of course, I don't know how much of that has to do with the fact that I use RES.
Logging out is the only thing I can think of that would cause you to keep seeing the new layout.
I have found that keeping the "Use new Reddit as my default experience" box unchecked on reddit.com/prefs results in almost never being redirected to the "new" site. The only exception is when someone uploaded a gallery - you have click on the "comments" link to see the gallery photos in oldreddit style.
This solution works for me on both mobile browser and desktop browser.