Template:HarvErrors

This section in a nutshell: Effective April 2020, CS1 templates (e.g. {{cite book}}, {{cite journal}}, and the other {{cite ...}} templates) behave like CS2 templates (e.g. {{citation}}) with respects to the generation of harvard-style anchors (see discussion). This means that adding |ref=harv to "long" CS1 citations is no longer required to use "short" citation templates like {{harvnb}}, {{sfn}}, and other {{harv}}-like footnotes.

If you see more yellow warnings like

Harv warning: There is no link pointing to this citation. The anchor is named CITEREFSmith2010.

than usual, those are caused by User:Ucucha/HarvErrors.js. To suppress/reduce these yellow warnings, you can use any of the following options

Using |ref=none in a citation template will suppress the warning from that template just like it would in a CS2 template. These warnings are only visible to those using some type of HarvErrors script, so you don't have to suppress them. But you can if it makes your life easier.
User:Svick/HarvErrors.js – Will only show confirmed errors, and won't show any warnings
User:Ucucha/HarvErrors.js – but also add window.checkLinksToCitations = false; to your common.js. Like with Svick's script above, this will only show confirmed errors, and won't show any warnings.
User:Trappist the monk/HarvErrors.js – Will show both confirmed errors and warnings, but only when {{harv}}-like footnotes are used. Warnings are additionally suppressed in sections where they are unlikely to be useful, like in the Further reading and External links sections.

If there still are errors after this, see how to deal with harvard-style citation issues. The documentation might be a bit out of date, but every piece of advice listed for CS2 (e.g. {{citation}}) now applies to CS1 (e.g. {{cite book}}) as well. If none of those options satisfy you, you can make a new script request at WP:SCRIPTREQ.