I agree with nearly every one else here that it was a dumb idea to remove "Learn Spelling" in the new Evernote. I'm just thinking, "What the heck?! Really?!" Removing the ability to "Learn spelling" and still include the annoying little red dots indicating that a word is misspelled, has to be a violation of fundamental programming 101.
Fortunately, there is a pretty simple way to "Learn spelling," at least on a Mac. I'm using macOS Big Sur and opening the Dictionary App (in the Applications folder) will allow me to type in the affected word and then in that very field where I type the word, I can right-click select, "Learn Spelling." You have to make sure the entire word is selected, not just click inside the word or else you won't see the "Learn Spelling" selection. Once this is done, it takes a few seconds for the red dots to disappear, but disappear they will.
You can also use a more round-about way, using more clicks and contextual menus to get this done in TextEdit or Microsoft Word, but the Dictionary app looks to me to be the quickest way to get it done, until the Evernote folks get their heads on straight!
Regardless of this annoyance, I still love Evernote. I've been using it extensively for years and have pretty much enjoyed it the whole time.