Bootstrap does not require jQuery. Not sure if ever but not since 3.0 and were are at 5.0 at this point.

    document.getElementById('some-form').getElementsByTagName('input').forEach(i => i.classList.add('error'))

That's also doing things "the right way" and not do-all selectors, but that option exists now, too:

    document.querySelectorAll('#some-form > input').forEach(i => i.classList.add('error'))

   const $ = document.querySelectorAll
   const addClass = (className) => (item) => item.classList.add(className)
   $('#some-form > input').forEach(addClass('error'))

    document.querySelectorAll("#some-form input").forEach(e => e.classList.add("error"));

Same thing of course applies to people who throw around the optional chaining operator without thinking to please TS.

Or would apply if one used a single selector here and document.querySelectorAll

The needless chaining here is a typical example of misleading jQuery code though. Looks as if it was there to prevent this issue.

  for (const input of document.querySelectorAll('#some-form input') {
    input.classList.add('error');
  }

In fact, this has an even earlier baseline browser compatibility than

  NodeList.prototype.forEach

Admittedly I'm usually using it in places where loading the extra library is not an issue, so if you're really going for slimline, then sure.

However, it does matter in interactive websites because if the js is light it'll load fast and execute fast and you'll have interactivity sooner. It may be the case that images can be loaded later or arent even visible on the home page. And fonts of course will be replaced once they are loaded and the defaults will be used while it loads so it's progressive in a way.

Then again, how many folks are using jquery these days? It might be more rare than I think.