The previous Core+ spell check functionality has been replaced with the spell check functionality in your given browser.
If you're unable to correct misspelled words in Core+, check and amend your browsers spell check settings. For Microsoft Edge, follow these steps to amend spellchecker settings;
Open Microsoft Edge.
Click the Settings and more three-dotted button from the top-right corner.
Click the Settings option.
Click on Languages.
Under the Use writing assistance header, select Microsoft Editor.
The spell-checker functionality for Chrome is still being investigated. Keep an eye out for CORE-153615 in future release notes.