It's especially annoying on pages that have big tables, where all the wasted space makes it need to horizontally scroll. Sometimes I wish that once a horizontal scrollbar appears, browsers would ignore max-width on that page from then on.

Did you disable cookies or something? Table of Contents sidebar hides and stays hidden across page loads, browser restarts, etc. Appearance sidebar is the same. Appearance sidebar lets you set the article width as well, with the default being slightly slim, but the wide option being the full browser width.

The only way I can get it to reset is to clear my cookies, so its likely that.

I have been using MonoBook for years (early 2023 I think), I see nothing like that.

It might have something to do with Wikimedia Foundation and WP:CANCER.

Is there some kind of extension to block their begging for donations banner?

I’ll answer deeper question “why” if you don’t mind.

Wikipedia itself has become corrupted and politically and ideologically biased in past years. It has been shown multiple times.

When you stop pursuing the truth and start pursuing an agenda - things get ugly over time. It’s a general pattern.